On the off chance someone else was seeking similar knowledge the best of my knowledge with uBlock:
To block general site chat
online-go.com##.chat-log
To block personal messages
online-go.com##.open.private-chat-window
To block game chat
online-go.com##.chat-container
And while there are some ways to try and force admin settings for uBlock as long as the user has basically any level of access into the PC it will be easy (and based on my experience of being one, students WILL learn quickly) to break it. Just downloading or connecting a “clean” portable browser will do the trick for example.
The only way I can imagine to make this even remotely “safe” would be to lock the computer into “kiosk” mode - which will run selected app in fullscreen and nothing else will be accessible and is quite easy to do for computers running win > 8.
and then lock settings (link in post above) and also restrict accessible domains:
NOTE that I have NOT actually tested this (not so bored just yet), but with a bit of tweaking it should be “safe enough” I think. And is not super hard to do if you have a bit of computer skill. (And if your school is not running XP anymore 
)