Hi Nancy, I didn't see how old David is. It is amazing what a few months can do for you. My daughter was in gymnastics (really, playtime with structure) for 3 terms, then we tried to do the without mommy gymn and it was a disaster, exactly what you are describing, 10 months later we are trying it again, and from the very beginning, she was completely differenrt and is often chosen as the example of waiting patiently. Then we went to skip, gross motor skills, and I could have sworn she was adhd. She couldn't keep on track for a minute, bouncing from one thing to the next. The next preschool readiness playgroup, she was wonderful.
All this is leading to something, I got to thinking about her behavior. For gym she was too young. For the playgroup vs. gross motor skills, she is too tired. I corrected this by changing the group she is in, and what a difference.
Anyway, make sure he is getting 11-13 hours of sleep a day/night, getting regular snacks, get him lots and lots of exercise, reduce sugar, higher protein, and see if those things help. I would trust the teacher, if she is experienced and not just out of school, she sees a lot more children than you do who have adhd.
But above all, follow your mother bear instincts and don't be deterred by what ANYBODY else says. If in your heart of hearts you think there is something wrong, then you need to get it checked. I would really watch the dvd that Marion offered, I have seen shows where a simple eye test of holding a pen in front of the face and moving it to the nose(if the eyes cross then normal, if one goes outward, then it is a sign of a problem) can diagnose a different condition with very similar symptoms. If you are worrying because everyone else is, and that's the main reason, then take a step back, breathe, and let your instincts take over. God bless, and good luck.