I think the issue is that to be diagnosed with ADHD, you are saying that the child has some sort of chemical imbalance in the brain, and it is this that causes the child to act without thinking, be very impulsive etc. Now, if this is a chemical imabalance, it can't be turned on and off, so the child cannot force him/herself to concentrate / think before behaving etc in any circumstance - and that includes school. So if the child is able to control his / her behaviour in school, it suggests that it is not a chemical problem, but something else.
Whenever I've had to complete school based questionnaires about this on my students, I'm usually asked to assess their concentration as well as their behaviour, and all the teachers are asked to complete these assessments... if the child is able to control his or her behaviour / concentrate for long periods then it is indicative that perhaps the problem is not ADHD, but may well be something else.
Now don't quote me on this, as I'm not an expert, but I had a child with severe ADHD in my tutor group, and had to do some research etc, looking at ways to support etc and this is what I remember to be the case from that child.
Personally, if its being questioned that your child might not have ADHD, be pleased. Ritalin is an evil drug and its not nice to have to have a child put on it. However, I do sympathise, because obviously you feel there is an issue that you need help with. I hope you can find the support you need, to help you find a way forward.