The school is effectively making the computer a terminal into their system. Their system, their rules.
I know the OP won't like this answer but the OP should buy their kid another computer for their non-school activities. Of course they could also complain to their school/district to change the polices. Personally, I'm used to it. I have a personal computer for my own use and a corp computer that's managed by corp and is locked down. I don't complain the corp computer is locked down (except when it gets in the way of doing my actual job). For all personal stuff I use my personal computer.