Personally I would leave the bit out about not understanding why you can be punished for not finishing a picture particularly as it is quoting your son. It makes it sound like he is questioning the teachers ability to set meaningful lessons (which of course may be TRUE but need not be said). I don't understand how I can be punished when I was trying as hard as I could would be better. Maybe outside the quotes as a paraphrase- as simply didn't understand being punished for making his best effort then asking for appropriate assistance (can't remember the official word).

Good luck.