Sound It Out or Get Kicked Out

Aiden has been really excited about writing lately. He's always asking how to spell words. He even asks people to "give me a word to spell." He's great at simple three and four letter words. It's so amazing to see him grasping it all. Yesterday he drew a picture and wrote a letter to his Great Grandma Moser. All the words run into each other, but he wrote it exactly like it sounds. I'm so proud of him. He's mighty proud of himself too.

Asher is loving preschool too. But I have a dilemma.
When I sighed him up for preschool he was completely potty trained. Well, that only lasted 3 weeks, and he's back to having accidents all day long. I mentioned to his teacher that he has regressed a little bit. (I didn't mention he's regressed completely.) She said to just send him in a pullup.
Here's my problem: he had a stinky accident last week at school. They were very polite about it. No one called me to come and change him, but he smelled horrible! He loves going to school and I'd hate to pull him out. But what do I do if he has another accident? Then they will really know he's not potty trained!

My plan is to keep sending him until it happens again and then he'll probably just get kicked out of school. Every morning before he leaves I make him repeat to me, "I will not poop at school." He thinks its funny, but I'm really serious. Poor kid. I've tried everything and he just doesn't want to go on the potty. Stephen thinks he doesn't want to be a big kid yet. This is his way of rebelling.

Any potty training tips would be greatly appreciated