What does Cassie mean when she says Stacey ""wheelded his way back into Stacey's good grades,"?

For the first part of your question, Cassie is inferring that her brother has forgiven TJ a little too easily. Cassie and Ma think TJ is trouble, they do not trust him. He has proven himself untrustworthy by his actions. Example, cheating on the test and allowing someone else to take the blame.