Hi!
We had the same issue last year. In fact, we secured a position at a toddler place and at a preschool and made a last minute decision. My son, who was 2 years and 9 months in September started at the local Montessori school. I should note that he had been potty trained for 8 months when he started school. He has seemed to thrive in the preschool environment. He has always been precocious--and he loves being challenged by his teachers and peers. He seems to always amaze us with the things he has learned and the teachers seem to enjoy teaching him. Most days he runs into class to start the day.
I would just offer a few things to consider:
Preschools often have open bathrooms (with no stalls)--I can say that the kids in his class that were not entirely trained have been subjected to negative attention by other kids in my son's class. Even my son knows every kid in his class that wears pull-ups. While we have told him that he should never tease or make an issue of such a thing, he does tell us that "Joe" teased "John" when he had a yucky poopy diaper.... If my child were not trained, I don't think I would have sent him--such teasing can really upset a child.
Also, my son is big for his age. He is as tall as most of the 5 year olds in his class. He can keep up with them. With boys especially, kids that can't keep up and jump and run and pedal as fast as the others can also be subjected to feeling bad.
Finally, I should note, that none of the teasing, etc. is tolerated by any of the teachers, but what we have found is that kids will be kids and you can't make one kid play with your kid...no matter what.
I hope that helps.
p.s., you can toilet train a child in 6 months if the child is ready...and if he isn't, then you won't be able to--imho.