Well, the next day we sent Chase to school in underwear - no pull ups at all - and lo and behold, he came home that day with only having had one accident at school! Since then, he has been just in underwear nonstop during the day, and hasn't had more than an accident a day! We're taking him to the potty on a regular basis - quite frequently - and this time he's actually cooperating and being willing to at least try to go each time.
What a huge reversal in behavior, and a grown up change for Chase!! We are thrilled!