There's a lot of subtle hints in the manga
Here's an amazing post on tumblr pointing them out
Part 1
http://msleilei.tumblr.com/post/147804297589/have-you-ever-put-any-thought-into-the-traitorspy#_=_
Part 2
http://msleilei.tumblr.com/post/147804297589/have-you-ever-put-any-thought-into-the-traitorspy
Really?! That'll be sad
with his electrification quirk Denki can pretty much disable all the sensors and allow the villains into school. As for knowing when All Might is at school, it's not hard to sneak into the staff room to take pictures of the schedule. In the attack at SJS, it was hinted that the information the villains received most likely came from a student's timetable.
does anybody have a clue as to who the traitor is?
all the evidence right now seems to point to denki