L.D.
When my son does it, he does it to relieve his anxiety. He has a language and social delay and being in a regular gen-ed class room probably made him feel a bit overwhelmed and anxious at times and that's when he would typically chew and suck on his shirt. What we did to save the collars on his shirt was develop a positive reward system. Each time I picked him up from school, if his shirt collar was clean and dry, we would put a sticker on a special reward chart I made up using a poster board and a lot of colorful shark stickers. If he earned 25 stickers, then we would buy him a special toy that he had chosen before hand (some kind of Animal Planet toy). When he earned 50 stickers, dad took him to the Shark Reef Aquarium - one of his favorite places in the whole world and one of his favorite people in the whole world. His shirt collars have been clean and dry for a while now but we are starting a new school year next Monday and none of the kids that he had in class last year will be in his class this year so I am expecting that we will have to reinstitute the reward system for him once again.
Hope this helps.