I don't think that would be a good idea. It would introduce an admin burden on the schools related to moderating/monitoring the sites. And they would more than likely overstep in one way or another, when enforcing their rules.
I was thinking state-administered. Public school enrollment would just be the precondition to access the program.
But sure, yeah, there'd be some admin time spent managing it. As with anything, there are plenty of reasons not to do it. It struck me as a low cost-to-impact ratio thing that could get kids into tech, but reasonable minds could disagree.
The only way it would work is if it was literally handled by the government, and the associated 1st amendment rules applied (so it wouldn't be moderated unless it was actually shut down by a court case).
It would result in rampant wildness and people complaining, but if you didn't do it that way the burden would be too high.
Cost would be negligible compared to a teacher's salary.
(1 teacher / 20 students) * ($50k / teacher-yr) = $2500 per student per year to fund teacher salary.
Compare that to $40/yr domain+hosting, which maybe 10% of students will use. $4/student-yr will not be the diffence between paying teachers probably or not.