My school has provided us some emails with a custom domain (not @gmail.com but @myschool.smth) and we as students are only able to login via gmail, however i obviously do not want that. What can i do to prevent that? I tried logging into the email via Thunderbird but i get blocked because my administrator doesn’t allow that.


You can read your student code, most of them do not have anything that compels you to use the services that are provided. If that is the case, refuse to use the service and make your position known, they’ll generally give you a workaround or if they say that’s how it is, you can ask for the part of the student code with the nonexistent objective reason why you must use it.
If you don’t want to be a squeaky wheel, there are technical ways of limiting the information loss, but the easiest is to not install anything and be conscious of what location, network, & device you use