Bingo. This sounds like a clear cut violation of 4th Amendment rights. You know the ones that are unalienable as in you cannot legally surrender them insomuch as they are bestowed upon a man by his creator not the state.
Also hair cutting for drug usage is used in spectral chromatography which can screen as far back as 6 months but cannot tell when the usage occurred nor for what reason. For example testing positive for opiates does not automatically mean illegal usage. It could have been for legitimate pain relief following dental surgery or injury.
I hope the school has good liability insurance because it sure looks like they are opening themselves up to legal problems and “It's for the good of the children” doesn't sound like a very good defense.
Indeed, many people here seem to think one forfeits one's rights (which is impossible, as they are inherent to one's person) simply because he is a minor or a student at a private school.