I was in our schools computer service team and they trusted the (competent) students way too much. I never misused those credentials but thinking about it now I could have done some hilarious stuff… Anyway. Even without too many permissions I did a thing or two to the computers. I once realised no computer had the BIOS password set. So I set one on a library computer to reserve it for me. Another time i realised you could take the whole network down if you connected two LAN ports directly with each other. That one was more on accident.