There are numerous reasons that drug testing should not be mandatory in a private school. The school must make a valid case that it expects to find evidence of a particular crime; otherwise, the invasive procedure is "unreasonable." Students' right to privacy is a crucial factor. Most high school students are minors, so the question of parental consent is involved, and parents bring the lawsuits.
The legal argument using the US Constitution's Fourth Amendment barring unreasonable search and seizure can be found in the 1985 US Supreme Court case New Jersey v. T.L.O, 469 US 325. This resulted in a landmark decision affirming students' rights to protection from such searches.
More recently, the 2002 Supreme Court case Board of Education of Pottawatomie v. Earls (from Oklahoma) interpreted federal law to allow "suspicionless" drug tests but acknowledged that state laws vary. In the Pennsylvania case of Theodore v. Delaware Valley School District (2003), the state Supreme Court used state law on search and seizure as the basis of its ruling that random drug testing of students is unconstitutional.
The applicability of state laws governing public schools to private schools has been widely interpreted. A 2002 opinion by New York's state school board association claimed they did not apply.