School officials are in loco parentis for minor students--i.e. they stand in for parents and have similar levels of control and authority during the school day or at school functions. So just as a parent could have a child breathalyzed without "reasonable suspicion," so, too, could the school. And schools have considerable discretion about discipline (and private schools even more than public, since there is no "right" to attend private school the way there is to a public school education), which means they could suspend him if they thought it appropriate, subject only to any limitations in the private school contract or agreement you signed in sending him there.