New Jersey v. T.L.O., 469 U.S. 325 (1985)
1. The Fourth Amendment's prohibition on unreasonable searches and seizures applies to searches conducted by public school officials and is not limited to searches carried out by law enforcement officers. Nor are school officials exempt from the Amendment's dictates by virtue of the special nature of their authority over schoolchildren. In carrying out searches and other functions pursuant to disciplinary policies mandated by state statutes, school officials act as representatives of the State, not merely as surrogates for the parents of students, and they cannot claim the parents' immunity from the Fourth Amendment's strictures. Pp. 333-337.

2. Schoolchildren have legitimate expectations of privacy. They may find it necessary to carry with them a variety of legitimate, noncontraband items, and there is no reason to conclude that they have necessarily waived all rights to privacy in such items by bringing them onto school grounds. But striking the balance between schoolchildren's legitimate expectations of privacy and the school's equally legitimate need to maintain an environment in which learning can take place requires some easing of the restrictions to which searches by public authorities are ordinarily subject. Thus, school officials need not obtain a warrant before searching a student who is under their authority. Moreover, school officials need not be held subject to the requirement that searches be based on probable cause to believe that the subject of the search has violated or is violating the law. Rather, the legality of a search of a student should depend simply on the reasonableness, under all the circumstances, of the search. Determining the reasonableness of any search involves a determination of whether the search was justified at its inception and whether, as conducted, it was reasonably related in scope to the circumstances that justified the interference in the first place. Under ordinary circumstances the search of a student by a school official will be justified at its inception where there are reasonable grounds for suspecting that the search will turn up evidence that the student has violated or is violating either the law or the rules of the school. And such a search will be permissible in its scope when the measures adopted are reasonably related to the objectives of the search and not excessively intrusive in light of the student's age and sex and the nature of the infraction. Pp. 337-343.

3. Under the above standard, the search in this case was not unreasonable for Fourth Amendment purposes. First, the initial search for cigarettes was reasonable. The report to the Assistant Vice Principal that respondent had been smoking warranted a reasonable suspicion that she had cigarettes in her purse, and thus the search was justified despite the fact that the cigarettes, if found, would constitute "mere evidence" of a violation of the no-smoking rule. Second, the discovery of the rolling papers then gave rise to a reasonable suspicion that respondent was carrying marihuana as well as cigarettes in her purse, and this suspicion justified the further exploration that turned up more evidence of drug-related activities. Pp. 343-347.