When first starting OP, there is no login information anywhere. The Windows clients are open and anybody can access them – how do I identify the teacher who wants to use the printer?

You need to enable OP users.
Launch the OP Web Console (Programs > OP) or enter https://server:1562/ in your web browser. Find your name on the Users page. Change your access type to Administrator and enter a new password. Go to the Settings page. Select the General tab and disable the checkbox Disable login password protection (OP Clients are disabled). Reload the page and enter your login name and password. Other users will have limited rights.