My son(7yo) has had this problem with chewing on things. Mostly his shirts. He doesn't do this all the time, but about 50-75% of the time. The teacher and I have noticed that when he does this, his behavior is worse at school.
He has gotten into trouble and lost some fun activities because of the this. Not to mention the negative affect of being around his peers. Plus the principal doesn't seem to care that the chewing and behavior are linked, and that my son can't seem to control it.