The method of connecting to server uses very long URLs that Firewalls block.
In my school, when we try to make a new Python3 repl.it, we sit watching the "Connecting" bar filling and empyting over and over.
Eventually it will sometimes connect, but it's very frustrating.
Checking with my network manager, he has analysed the traffic and says that a connection is being attempted with a very long URL string, containing lots of data. He says the firewall won't let this through.
He also say that this is not a system that is unique to our school, so there must be others experiencing the same problem.
This sounds like something fundamental to your architecture, so I don't know if it's easily fixable, but it means I probably won't be using it in lessons.