Just to start off this forum heres a hack me and my friend did a while back.

We gained access to the teckys Administrator account at our school we then added new users all over the school with random names then a few weeks later we were extremely bored in our ict lesson so we logged into one of the admin accounts opened command prompt and typed the command:

netsend * hello

which opened message box on every computer in the school (over 100 computers) saying hello,
as we were laughing our heads off n everyone was screaming stuff like "bloody geeks" then the server admin came in took us out the lesson and a tecky took the computer we got suspended for 1 day lol but the funniest thing was they had to ask us to tell them how to get in to the user accounts that we had set up lmao