I can't remember if it was Kurt/Blaine or Kurt/Puck. Kurt as a teacher that is cruel to him in class and gives him bad grades because Kurt is gay. One paper Kurt received back the teacher had written comments on it that were completely pointless.
Kurt and Blaine(or puck) go and speak with another teacher about this, I believe either Mr. Shue or Ms. Pillsbury.
Edit: Found! Link in comments!