'Okay, thanks!'
Lianne heads to track once gym's done, and explains to her coach that she's falling behind in classes and needs more time to focus on homework and studying. He nods, says something commiserating about the modern homework load, and assures her there'll be a place on the team if she wants to rejoin later.
She thanks him, and heads for the school library until the activity buses arrive (might as well get started on that whole 'responsibly focusing on classwork' thing).
Her mom doesn't seem to know how to react to Lianne dropping track. Eventually she settles on asking Lianne's sister to make sure she spends at least two hours studying after getting home each day she would've had track. (Which, ugh, but her mom doesn't threaten to make her drop the 'scouting' group, so better than the worst-case scenario.)