Question

in what anime does the main character thought he got isekai'd but he was actually told the administrator of this violent school

Answer 1

The anime you are referring to is probably "The Misfit of Demon King Academy" (Maou Gakuin no Futekigousha). The main character, Anos Voldigoad, is reincarnated and believes he has been isekai'd to a different world. However, he later discovers that he is actually the Demon King and the administrator of the demon school where he enrolls.