I mean we're a general purpose computing platform, if we allow you to build and host a website, of course it's possible to build a proxy. It's essentially impossible to stop that. However, I doubt that's the main driver for the blocking.
I can see why schools may not like it though, apart from just the proxy stuff, i imagine kids use repl.it a lot to do things their teachers would find unproductive.
Just thinking back to the kind of crap i would do on school computers even back in the early 2000's, i can't even imagine what kinds of things kids have come up with these days to fuck around on a computer instead of doing their work.
Like i said though, i think repl.it can be a great tool and resource for students, it seems like it may have to involve more direct effort from you guys in working with schools to make this happen though.