Courts have consistently held that students on school property have a lowered expectation of privacy.
The relevant phrase of the Constitution being, of course:
''...nor be deprived of life, liberty or property, without due process of law; ...''
I should think that virtually any court would rule instantly that ''due process'' consists of rather more than some a&&wipe principal's diktat.
Don't the rulings apply to school supplied lockers?
I would expect a cell phone to be treated as a personal diary, not to be searched with out a warrant.
Best thing to do is use phones that can hold the data under some secure form of encryption.