So I get that Tommy didn't write the poem and when Carrie said "it's beautiful" the teacher started making fun of her but when Tommy said "you suck" was he referring to the teacher or Carrie?
The teacher. I think this scene is important as it establishes Tommy as a decent person.

I think he was saying it to the teacher. The teacher was basically making fun of his own student, and as we see later, Tommy becomes fond of Carrie at the end. I think this scene was foreshadowing of his later feelings.

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