I have found the extracurriculars, like piano, fill the void. If she is doing math so far ahead, she probably has good ability in piano and the work required in doing the piano at a high level, takes away the boredom at school. You said you cannot add any more activities, but that is a real way I have found to deal with it. Also DD takes Chinese out of school, chess and spanish in school. So the extra languages and piano really challenge her, though we do accelerate the math at home.