What did Stacey do to help TJ from getting into trouble?

I think that you are referring to Ch.4. TJ makes a "cheat cheat" to use in Mama's class. Just as mama is coming towards them T.J. passes the paper to Stacy. Stacy takes the blame. He is too honourable to tattle on TJ. Stacy, however, does catch up to TJ and beats him up after school.