It may sure be something to stimulate. I still chew on clothes sometimes. When I was a kid I chewed much more in addition to eating pens, pencils and other things. Im very in need of tactile stimulation, and the chewing part (which are now only pens, coca cola corks, chewing toys and so for the most), are tactile stimulation for my mouth.
Some children do this because of sensory issues some slap on objects this is called stemming it calms their nerves a therapist were I work offers the advice of getting him a viberating seething toy when he starts to chew on his shirt give him the toy say his name and show him chew on this not your clothes
This can either be a nervous reaction to his environment at school or some form of emotional relaxation thing that calms him. Either way I would try to find other releases for him to do that helps/eliminates the chewing of his shirts like the rubbing of the thumb and pointer finger is a way (according to therapist) so help sooth the mind when under stress. How old is he? Judging by age their can be many ways you can substitute it for doing something else.
Yes, this is typically a way that a child is trying to soothe themselves if anxious or nervous. My son has sensory integration disorder and he does this as well as oral soothing is very common. You can offer a coffee stirrer or straw to chew on. Or they make things you can get on therapy websites that are rubber and made for chewing. Also rubber tubing around the wrist can work or they make something that slips on the tip of a pencil. Best to figure out what the root cause of the anxiety is. good luck