The Fourth Amendment to the U.S. Constitution protects citizens from unreasonable searches and seizures. This means that for a search to be legal, it has to meet the "reasonableness standard."
The circumstances around the search must be measured against the overall intrusiveness of the search and the interests of the government.
If a police officer cannot prove that a search was necessary, then it will always be considered unreasonable.
In many instances, a warrant is needed to perform a search. Whether or not a warrant will be issued is dependent on the probable cause statement provided to a judge. If the judge believes there is sufficient probable cause for a search, he or she will sign off on the warrant.
However, when are warrantless searches legal?
When considering a search of a student's vehicle at school, a warrant is not needed. If a school employee is a person conducting the search, then he or she only needs reasonable suspicion for a search.
If a police officer were conducting the search, then the probable cause is needed. In many cases, a school resource officer is a police officer. If a search of a student's vehicle is based on law enforcement information, then the probable cause must be present. If the search was directed by the administrator of the school, then reasonable suspicion is needed.
A vehicle can always be searched on school grounds if there is something illegal in plain views, such as drugs, drug paraphernalia, or a weapon. The school should have a policy that addresses when a vehicle can be searched. This will help in case the question of liability occurs.
As you can see, the laws regarding search and seizure can be complex. An experienced attorney can help you determine if the evidence gathered from a search was gathered legally.
Source: ThoughtCo, "Search and Seizure in Schools and Fourth Amendment Rights," Derrick Meador, accessed Dec. 08, 2017