Any ideas or tips? (PT related)

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O has been doing awesome with potty training.  He is pooping and peeing on the potty, we have had no accidents in several days, and he is only in Pull Ups at naps and nighttime.  So far, so good  

The issue is preschool.  He goes 2 mornings a week from 8 -12 and he will not go there.  He goes through 5 or 6 pairs of underwear and shorts, until they finally run out and put him in Pull Ups.  I have no idea why, they are taking him every 20 minutes, we remind him that he has to tell his teachers if he has to go, he needs to keep his undies dry, and so on.  They have little bathrooms in the class room and other kids go, he has no issue going in public restrooms.  Any ideas or tips on how to get him to go there?  I feel so bad for the teachers for dealing with this, and frustrated that I have to wash so much laundry :-p