The following form is just a suggestion from PAGE if you need assistance initiating a conversation with your child's teacher:
I'm writing about my child, ____________. I appreciate all the good work you have done with him/her this year, but I feel that s/he would be more engaged if provided with more challenging work. For instance, where you currently teach _____________, perhaps my child could have __________ instead. This would enable my child to learn at his/her own level and make him/her a more productive member of the class. Can we meet to discuss this prospect?
(You should adapt this to your own needs - P.A.G.E. hopes you find it a helpful way to start this conversation.)