Ok, I'll give you the tactic.
This is for teachers entering grades to online system, like JupiterEd mostly used in USA.
You setup keylogger to the teachers computer.
When he/she enters his user name password to online system, keylogger will save it.
Get the keylogger saved files
Find user name password of the teacher
Easy. The easiest way is this if your school isn't protected by cameras.
If protected by cameras, you have to delete camera saved files. I can't say the method bcs It changes depending on the system your school uses, and who checks the cameras.