It sounds like he's bored. I would be curious to see the impact on 'focus' of more challenging material. How does he behave when doing more academic activities at home? If he reads happily, can he do so in a sustained manner? If he enjoys math puzzles, can he persist at a problem? To me, those would be more appropriate measures of his ability to attend than occasions where the teacher (in my opinion, inappropriately) leaves the class.

What is to give light must endure burning.