Can't send POST requests to my website

Most likely. Perhaps your school proxy has a sort of adblock-related function which blocks some scripts from loading, and prevents your website from functioning correctly.

The browser could also be an issue, did you try connecting on the same network (your school-managed one) on a different browser? If you manage to connect via a different browser an extension or setting in the browser that doesn’t work could be the issue as well.

Interestingly, I find out I cannot connect via HTTPS to your website because I get an SSL protocol error. Is anyone else facing this?
The following article can help if so: