My five year old has started pooping his pants at school

My five year old son has started pooing his pants at school. He has been in school since Sept 2009 and has been fine up till now.
The teachers will stop me at the end of the day to advise that he has pooed in his pants today. They said he will not say anything about it, he will be playing as normal, they only know it has happened when they notice the smell. I have asked the teachers about it and they say he used to use the toilet fine, and still uses it for wee's.
I have asked my son why he does not use the toilet, is it that other students make fun of him or the smell, he says no. I have asked if it hurts when he goes to the toilet and is this why he does not like going, he says no. There has been no changes around him or at school so it can't be to do with that.
I have tryed acting angry about it, and telling him he must go to the toilet, but find myself being mostly sympathetic, my poor little boy, i don't want people to think bad of him. I must add that he has done it a couple of times while he has been at his Nans playing, is it because he fells too busy and does not want to miss anything going on. Its just the fact that he would quite happily sit with dirty pants till someone notices.

[Rucnfun]

hi, could it be that someone has sexually abused him? i know thats the last thing we want to think of, but sometimes when children are then they start wetting the bed etc.

[MUM187]

Thanks for your comments, i know you are trying to help. Its just simply that he is having accidents at school i don't think we need to mention anything like that. This was not the feedback i was expecting, maybe some medical advice on what to do about it not an outrageous question about my child being sexually abused.
Any medical advise or advise from mums whom have experience with this type of thing (a 5 year old pooing his pants that is) your advise would be helpfull.