Only can visit certain websites

Don't know, you have not told us what browser, what url, what OS, what error messages.

Maybe the school website is doing some mac filtering.

Maybe you are running some third party app that claims to protect etc.

Maybe you installed a browser extension that is preventing the site from being accessed.

Maybe you have installed proxy or vpn.

Maybe the browser you are using is incompatible.

A couple of things you could try:

1. Try shutting down Safari if Safari is the browser you are using. Then launch Safari while pressing and holding the shift key. This will start Safari in a clean state.
2. Try booting in safe mode as a trouble shooting step.