Last thing we heard it was the teachers concluding that none of the students are guilty, and that was a few arcs ago already. Tbh at this point I wouldn't be surprised if it was just AFO being able to sometimes look through Deku's OFA and relaying info like that
it's possible that the traitor is loyal specifically to One For All, rather than to the League of Villains. thus, while he was in prison, they weren't super active