I have to say that it doesn't sound like ADHD, as that would be obvious at home as well as at school. It does sound like he is attention seeking, or doing it out of boredom! Maybe when they are sitting on the mat, he finds that is boring and wants to learn more? Perhaps he does need something a little harder for him to do, which will take him longer! It can be difficult for a teacher to gauge where everyone is at, perhaps ask what she thinks?