Peeing in Pants

My son is 3 1/2 and is potty trained. He started nursery in November 2007 and was fine going to toilet at school and home suddenly he started doing accidents in his pants mostly at school. At home we make sure he visits toilet after 2 to 3 hours and he is fine, he some times shouts and says I need a vee and runs to the toilet but some times he is so busy with what he is doing that he hold it and untill he finishes what he was doing and also refuses to go because he dont want to.
We as a parent dont had this problem with our elder son and he was fast in picking things up but we dont understand what is the problem with this one either it is just lazyness or its something else.

The nursery teachers also got tired of him and are saying that if he is going to do the same they will put him to parttime from fulltime and we dont know if that is the only option because according to them they dont have the facility to keep an eye on him and keep asking him to go to toilet and if he has done an accident then keep cleaning him.

We dont know what to do with him and how to make him visit the toilet on his own without delaying it and without being lazy. Please give us some suggesstions.

Regards
Umer