Just played through the end of chapter 1 again. I remember Maya saying the other note on sensei's wall signed "MM" not being from her. The signature looks alot like Makoto's, too. Has there been any indication for why that I missed or is that something for the future?
This case still remains a mystery. Makoto is an very unlikely candidate imo because the note looked very vague and ambiguous in contrast to her uptight nature; plus, she doesn't have access to Sensei's bedroom up until he decides to invite her for some fun.
Maya states the note looks like something she'd have said (and to be honest the note looks like as if a version of Maya was trying to help a version of Sensei complete "There Is Nothing" but ultimately fell short because she couldn't even login).
So personally I believe the note is either still from Maya (she said she would've remembered but I am taking it with a grain of salt), or someone/something impersonating Maya to figure out Sensei's password (but currently there isn't any character that fits the description though)