My son is about to be 7 and he also has this habit on and off...not quite to the point of chewing holes in his shirt, but they will get all soaked in the front and stretched out. It is definately stress related for my son and if your son is just starting school, then he is probably using this as a stress/anxiety response. You may try letting him take something small and special (like a favorite little toy or marble) to school to keep in his pocket so he can feel that when he is nervous? Maybe let him chew some sugarless gum to keep him busy when he is having a hard time with it. I do know that the more you repremand him about it, the more anxious he will become when he feels he is dissapointing you and the harder it will probably be for him to stop because the anxiety may make him want to chew more! Good luck and try to be patient with him, even though inside it may make you crazy. My son has, for the most part, stopped this habit on his own over the last year.