What can the school do or see through the google school accounts?

Provided it's a G-Suite account (and it almost certainly is, as this is the only way they can manage the accounts unless you gave them your login credentials), they can see all data associated with that account, including but not limited to emails, search history, Youtube watch history, and numerous other things (for example, browsing history if you've logged into Google Chrome with this account as your primary account). However, unless you explicitly grant access, they do not have any access to data on your other Google school accounts (with the possible exception of the fact that you have logged in with other Google Accounts on the same device, but probably not what accounts those are).