For several years it has been the law in the United States that if the police illegally seize someone, then any contraband that the police find on that person--such as guns or drugs--cannot be presented at that person's trial. This is sometimes referred to as the doctrine of the "fruit of the poisonous tree."
However, when the police search a student's property at school, such a search may not be illegal even though the student's behavior does nothing to arouse the officer's suspicion. Thus, in one Florida case, the court ruled that it was legal for the police to search a student's purse even though they did not suspect her of having done anything illegal.
In that particular case, the court said that the search of the student was an administrative search which had the purpose of deterring students from bringing drugs and dangerous weapons to school. This type of search differed from those searches which are intended to penalize individuals who are found with contraband on their persons. Therefore, the court ruled, it was legal for the police to search the student's purse.