I like the idea of having kids scan their ID to get into a restroom. Restrooms are one of the hardest places to keep safe in a school. (You can't install cameras in them and faculty never go in because they have their own restrooms.) It's a prime location for bullying, vandalism and drug deals. Because schools have a primary responsibility of keeping kids safe, courts have given a wider latitude to school officials concerning the right to privacy. School officials do not need probable cause to search a student's car, locker or backpack. They only need reasonable suspicion that the student is breaking a school rule. The right to privacy has to be balanced against the school's interest in safety and student discipline. It's obviously too intrusive to install cameras in student restrooms, but having kids scan their ID does not strike me as too intrusive, and it gives a record of who was in the restroom at what times.
How would this be any different from just installing a camera outside of the bathroom? Besides, this just tells who goes in and out, not what happens in there.