M.S.
Not pooping on the toilet at this age is NOT a discipline issue at all. He may not be ready. Many kids do NOT poop during the day while at school. This is VERY normal.
The pull up situation - WHY would he be left in his pull up ALL DAY?? Do the teachers know that you are sending him off to school in one? Maybe he should wear underwear, you pack extra clothes and when he has an accident, they change him? He is more likely to stay dry this way, especially since preschools have the kids on a bathroom routine. I wouldn't worry about him not pooping at school. Give him ample time after breakfast to relax and try to go - or maybe it's an evening thing for him? My son goes almost everyday at night or shortly after school (he is very regular) but no way will he go at school or anywhere public - that is just his thing.
Also don't confuse night time potty training with day time. They are different.
Lastly - nothing to feel guilty about (except being OK with him being in a wet pull up all day) other than that, you didn't wait too long - it's always best to wait until they are ready. Some kids are ready at 2, some at 4! Each kid is different.
Pack the extra clothes/underwear, mention to the teacher that he is newly pottied trained and I'm sure things will be fine. Also, if you notice that he starts to stay dry at school but has accidents at home - try to keep the same routine at home that he has at school, regular trips to the bathroom, etc..
One more thing - try not to make any of this a big deal. Kids will respond to the stress and the last thing you want is him holding in his poop because he is afraid to go. This can lead to some serious issues down the road (speaking from firsthand experience here).
Best wishes!
M.