Help with VNC (via school Wi-Fi) without consent? Opsec
The only way they are going to be able to watch your screen is if you have installed something specific to allow them to do so or they have a password and you have screen sharing (and or remote management) turned on.

So turn off screen sharing (System Preferences > Sharing. Uncheck screen sharing and remote management) and change your password and any other passwords on the system in System Preferences > Users &amp; Groups.

This all presupposes this is YOUR Mac and not one provided by the school. If it is provided by the school they likely have it locked down so you can't make those changes because the computer does not belong to you.