He kinda already did by being completely honest with Mikan. Since up to that point he was hiding his feelings.
Also, if he said “I used to be able to read minds but I can’t anymore” they probably wouldn’t believe him. And he wouldn’t have a good way to prove it.
So she won’t confess to him because she’s worried about distracting him from his studies for the entrance exam, but he’s already done with the exam? Lol.