That post-it note in the end is going to be a plot point so that manabu finally realizes that was Amando who left the message on the eraser. I hope this happens in the close future of this manga, we are coming dangerously close to 300 chapters and I wouldn't want this series to become an anti-development manga like Komi-san
if he achieves sage mode, his test scores will probably shoot up to max. all he needs is to be with Megumi. once he realizes their love for one another, then he will ascend to the next level.