But that would only work if i work on the computer. A student was told the admin password and i need to know if he shared that password with other students. So i don't want to collect all laptops and was hopping that the mdm would be able to show which user has which status (admin or standard)
That's true. MunkiReport might be a good solution for that.
Or, if you want to brute force it... use ARD to run the command against all your computers at once. Do it while the little heathens are in a lab class where all the laptops are open. Then capture the output, drop it in a text editor, and then search for anything out of the ordinary.