Out of our six, we have three blurters, two non-participants, and one just right. Seems like it's hard to participate just the right amount to please some teachers. Maybe you could ask for a guage of what the teacher expects...one comment per subject? raising the hand three times a day? This would give your child an idea of what is expected.
Also, I wanted to add that some schools have adopted the idea that (just like job performance reviews) there has to be an area of excellence, and an area of improvement. With my non-participant children, they excel at everything related to school so I sometimes think this is one area where the teachers feel justified listing an improvement.
In fact, one of my non-participants has told me she raises her hand often, but is not called on because the teacher favors the more problematic children, or children who are daydreaming.
