My son did that in 1st grade. It must be a boy thing/phase, lol. He even chewed off the top button on one of his shirts (it was a polo style). Have no fear, Mom, he will outgrow it before you realize it. By next year he will have figured out something else. For my son, it was chewing his pencils... :(
He doesn't do that anymore now (he's in 7th grade now), probably because he uses mostly mechanical pencils, and they don't "chew", lol.
It's just his way of relieving stress right now. He will find something else. My son also would tear up little bits of paper and untwist paper clips. Always something with the hands.... Maybe if you got him one of those stress ball things (the stiff ones that have a face and hair..) for him to squeeze... he could keep it in his desk or backpack or jacket pocket, and pull it out and squeeze on it when he feels stressed.
Or wait it out... he will stop in his own time. One year of ruined shirts is probably all you have to worry about...