I would look at the Supreme Court case, J.D.B. v. North Carolina. Here's a quick run-down:
In 2005, A North Carolina 13-year-old special education student named J.D.B. was questioned by police at his school about a stolen digital camera. A uniformed officer escorted the student to a school conference room, where two officers and two school administrators questioned him for 30 to 45 minutes. They started with small talk to make him comfortable, then encouraged him to “do the right thing,” and then finally warned him of possible juvenile detention if he didn’t confess. J.D.B. confessed to the crimes, but later sought to have his confession thrown out. He argued that because he was in police custody when he confessed, he was a minor whose parents were not informed of the police interview, and he was entitled to Miranda protections. In December 2009, the North Carolina Supreme Court ruled on the case. They ruled that J.D.B. was not technically in police custody when he was interviewed. Therefore, he was not entitled to Miranda protections. The family appealed this decision to the U.S. Supreme Court.
The decision was 5-4 in favor of J.D.B., but it's a tricky decision. They clearly said that a determination must be made as to whether or not the student was "in custody" (what MarkEsQ is talking about above). They also said that in determining what constitutes "in custody," age matters. Justice Sotomayer wrote for the majority, "It is beyond dispute that children will often feel bound to submit to police questioning when an adult in the same circumstances would feel free to leave. Seeing no reason for police officers or courts to blind themselves to that commonsense reality, we hold that a child's age properly informs the Miranda custody analysis."
Warrants aren't needed in public schools for most searches. The Supreme Court has consistently ruled that rights in schools are limited to protect the two main concerns in the school environment: safety and education. School authorities can search lockers, backpacks, cars in the parking lot, and can confiscate cell phones. They can bring in drug dogs, and drug test those who volunteer to participate in extracurriculars.