SANFORD, Fla. -

Hundreds of students walked out of a South Florida high school, and thousands of protesters are expected in Sanford on Thursday to call for the arrest of a neighborhood watch captain accused of fatally shooting an unarmed 17-year-old South Florida boy.

Police said George Zimmerman, 28, fatally shot Trayvon Martin last month. Zimmerman claimed self-defense and was not arrested or charged in the case. 

Hundreds of students left Carol City High School, which Martin once attended, and walked down streets in the area in support of Martin and in protest of the lack of an arrest.

"We decided to do this earlier, around 11, because, you know, we want justice for Trayvon. We were in the cafeteria, and everybody was talking about walking out of school after first lunch," said one student.

Students said the protest began a rally outside the school and grew into a walkout, with the protesters walking through the streets of Miami Gardens.

The students marched peacefully, arriving at the Betty T. Ferguson recreational complex at Northwest 199th Street and 29th Court. Some teens held Skittles and iced tea, items Martin had bought at a convenience store shortly before he was shot, while others carried signs saying, "Justice for Trayvon."

VIDEO: Walkout creates trouble for police

"He's one of us. He's a teenager who hasn't even lived his life yet," said one Carol City High student. "It just hurts us because that could have been us, too."

"Trayvon was just such a nice and loving kid. We're just out here just to protest and nothing more," said another student.

According to the Miami-Dade School Board, the school's principal did not give the students permission to leave school. They were only given permission to go onto the field.

The school board also said that Martin's mother called the school to say she does not endorse student walkouts. She urged them to attend organized rallies, sign petitions and pray.

On Wednesday, Martin's schoolmates at Krop Senior High School in Miami Gardens donned hoodies in remembrance of Martin, who was wearing a hooded sweatshirt the night he was shot. A "Million Hoodie March" was held in New York City by protesters demanding Zimmerman's arrest. 

The protests will continue at Thursday in Sanford, where the Rev. Al Sharpton was expected to join a "National Rally for Justice for Trayvon Martin" at 7 p.m. at Fort Mellon Park in downtown Sanford. 

Sharpton's mother died Thursday morning, but he said he still plans to attend the rally, despite his personal loss.

U.S. Rep. Frederica Wilson also plans to attend the rally.

The rally was originally planned for a 400-seat church, but officials decided to move the rally to make sure there would be room for everyone. 

Martin's parents also will attend the rally, after meeting with U.S. Justice Department officials.

At the Antioch Missionary Baptist Church in Miami Gardens, which Martin's family attends, 50 to 100 members gathered Thursday morning to catch a bus to the rally in Sanford.

VIDEO: Group travels to Sanford

At the time of Martin's March 3 funeral, members said they did not know what had happened, but now that they do, they want to be a part of the rally for Martin.

"My son is 17. He wears hoodies. There's no law against wearing a hoodie, and then he walks home from school. You know, you worry about going from home to school, school to home, home to school. If anybody's trying to attack him, if he's in trouble, he's innocent," said parent Tekah Hill, who is going to Sanford.

"It happens so often, and nothing is done about it," said Ruby Mosely, who is also headed to Sanford. "With the Stand Your Ground Law, it's so broad and so open that everyone feels like it's OK to kill and walk away with it, and no one gets charged with anything. This time, it's not going to happen anymore, because people are tired."

Some church members said they also plan to circulate a petition to protest the "Stand Your Ground Law."

Some question 'Stand Your Ground Law'

Florida is among 21 states with a "Stand Your Ground Law," which gives people wide latitude to use deadly force rather than retreat during a fight. The self-defense law helps explain why a neighborhood watch captain has not been arrested in the shooting death of an unarmed teenager. 

The Florida law lets police on the scene decide whether they believe the self-defense claim. In many cases, the officers make an arrest and leave it to the courts to work out whether the deadly force is justified. In this case, however, police have said they are confident they did the right thing by not charging 28-year-old George Zimmerman. 

The teenager was black. Police say Zimmerman is white; his family says he's Hispanic. The shooting's racial overtones have sparked a national outcry and debate over whether the shooting was warranted. And like in many self-defense cases, two sides of the story have emerged. 

Zimmerman told police he was attacked by 17-year-old Trayvon Martin after he had given up chasing the boy and he was returning to his truck. He had a bloody nose and blood on the back of his head, according to police. Martin's family questions Zimmerman's story, and believes if their races were reversed, there is no doubt a black shooter would be jailed, even if he claimed self-defense. 

"They are making it look like Zimmerman is the victim and their son is in the grave," said Benjamin Crump, attorney for Martin's parents. "It's about equal justice." 

The Justice Department and FBI have opened a civil rights investigation, and the local prosecutor has convened a grand jury April 10 to determine whether to charge Zimmerman. 

Based on what's publicly known about the case, Michael Siegel, a former federal prosecutor who now directs the Criminal Justice Center and Clinics at the University of Florida law school, said it appears Sanford police were too quick to decide whether Zimmerman should be charged. If the evidence is murky, he said the usual practice is to make the arrest and let the court system sort it out. 

"The law has definitely shifted and given a signal to law enforcement to be more careful," he said. "But in a case where the self-defense claim is weak, you would think they would do their job." 

In a statement released Wednesday, Sanford Police Chief Bill Lee insisted his officers were "prohibited from making an arrest based on the facts and circumstances they had at the time," including physical evidence that supported Zimmerman's self-defense claim. 

"The Sanford Police Department has conducted a complete and fair investigation of this incident," Lee said, adding that it's now up to prosecutors to determine whether to bring charges. 

Late Wednesday, commissioners in Sanford, a city of 53,000 people outside Orlando that is 57 percent white and 30 percent black, voted 3-2 to express "no confidence" in the police chief. 

Under the National Rifle Association-backed Florida law passed in 2005, Florida, unlike most other states, grants immunity from prosecution or arrest to suspects who successfully invoke the "stand your ground" claim. And if a suspect is arrested and charged, a judge can throw out the case well before trial based on a self-defense claim. 

That happened Wednesday in an unrelated case. A Miami judge dismissed a second-degree murder case, citing the Stand Your Ground law and ruling that 25-year-old Greyston Garcia's testimony about self-defense was credible. The Miami Herald reported that Garcia was charged after chasing down and stabbing to death a 26-year-old suspected burglar in January.

Still, it's not enough for Zimmerman or anyone involved in a confrontation to simply claim innocence based on no duty to retreat, said Fordham University law professor Nicholas Johnson. 

"By the Florida law, he is not relieved of the traditional and basic requirement of showing that he fairly perceived an imminent deadly threat," Johnson said. 

Crump, the Martin family attorney, said the teenager weighed about 140 pounds and was carrying a bag of Skittles and a can of ice tea he had bought at a nearby convenience store when Zimmerman began following him in his sport utility vehicle. Zimmerman, meanwhile, weighs around 200 pounds and was armed with a 9mm semiautomatic handgun, which he had a permit to legally carry. 

"So the facts that have come out that I have become aware of, would tend to indicate he should not be granted immunity," Roger Weeden, an Orlando defense attorney closely following the case, said of Zimmerman.

State figures indicate that justified use of deadly force by private citizens is on the upswing. 

Florida Department of Law Enforcement statistics show that before the law was enacted in 2005, there were about 13 justified killings each year by citizens from 2000 to 2005. Between 2006 and 2010, the average has risen to 36 justified killings each year. 

Some state lawmakers are already questioning whether the law should be revisited. 

State Sen. Chris Smith, a Fort Lauderdale Democrat, said he is preparing a bill that would not allow a self-defense claim in cases where the shooter appeared to provoke the victim. That could have be a factor in the Martin case, where 911 calls and other evidence shows that Zimmerman was following the teenager in his vehicle and approached him aggressively despite specific instructions from police to back off. 

"Stand your ground appears to be giving suspects better protections from arrest and prosecution than increased security measures for the citizens the law was originally intended to protect," said Smith, whose bill would also limit legal use of lethal force to places such as a person's home, car or workplace. 

Lee, the police chief, said in a statement that the police dispatcher's "suggestion" to Zimmerman that he did not need to follow Martin "is not a lawful order that Mr. Zimmerman would be required to follow." 

"Mr. Zimmerman's statement was that he had lost sight of Trayvon and was returning to his truck to meet the police officer when he says he was attacked by Trayvon," Lee said. 

Republican Gov. Rick Scott, who was elected after the law's passage, said he's open to suggestions if the Martin case illustrates problems with it. 

"If there's something wrong with the law that's in place, I think it's important we address it," Scott said Tuesday. "If what's happening is it's being abused, that's not right."