Bitchy teacher involved somewhere, I'd bet.
Who would know/care that the kid was alone in a classroom, happens all the time.
Only factor would be if it was a science/AV/computer lab or such that needed to be locked for security/theft reasons.
Also, is the kid a untrustworthy troublemaker by history?
Most likely some bitchy teacher got bent out of shape and reported him to the admin. Admin had to do something to show they 'backed' the teacher.
If so, hope it bites the district in the ass.
BTW, 17/18 YO senior. Many cases they can be more mature than some of the teachers, especially if they work after school/weekends in a job with some responsibility. i.e, the business owner trusts them and treats them like an adult.