Doing well but need advice

fiasco

 I need some advice by Tuesday.  DS had his first day of school on Fri (3 yr old classroom, same school as last year.)  There is definitely no rule that children have to be potty trained by this year and they are totally open to helping with PTing.  Friday after school (it's just 3 hrs in the a.m.) we started our first real attempt at PTing and so far it is going awesome!  He already got 10 stickers for making it to the potty on time, has told us he needs to go in the middle of playing, ran to the potty & made it on time, and even went on a public toilet today.  He had two accidents today. My method was straight to undies, except during nap & night when he wears diapers.  I haven't even bought any pull-ups and not sure if I will.  So on Tuesday when he goes back to school, I should send him in undies right?  Obviously with changes of clothes.  Diapers or pullups would be a step back?