I've posted about this before, in chat. My 4 year old has global development delay and suspected autism. He was also a prem baby and was lucky to survive. He is cognitively about 2.5-3 years old
He has always put things in his mouth. I manage to mostly stop this at home with constant supervision and toddler proofing. At school he has a lot more freedom so he is eating sand, glitter, craft bits, carpet fluff etc. I've talked to the teacher, the senco and friends who work in education. They all say it's normal behaviour for a 2-3 year old and it is in line with the rest of his development. All toddlers do this and he will grow out of it.
I'm still worried though. He goes through phases of being constipated. We get him back to normal when he is at home at the weekends and then he starts having problems again. Surely this can't be good for him? Sorry for slightly disjointed post with not a lot of detail, trying to type with one eye on ds.