Also keep in mind that teachers are taught that "when mixed ability learners work together they both learn - but the more advanced one learns the most!". Which may be true, to a point, I'm told this is research based, but I am willing to bet this refers to children that both fall within the "normal" range for grade - ie there is a gap in ability, not a yawning chasm. You're cross that he's being used as a teaching aid, teacher probably sees it as providing maximum learning opportunity for YOUR son...
