Some kids will not poop at school. Not surprising, considering how many adults won't have bowel movements in public restrooms either! This can simply be the result of a subtle shift in his "time of day." If he used to have bm's in the afternoon or evening and now all of a sudden it is coming during school hours, this could be the cause of this.
It sounds like the teacher is being good about providing gentle reminders. I would take a peek at the bathroom one day and see if it is intimidating. I would also talk to your son a bit more about it. It can also be due to something like someone making a comment about the bathroom being stinky one day after he went. 6 year olds can be very sensitive.
If he is otherwise happy and not showing signs of anxiety, I wouldn't be too concerned about a greater underlying problem. I would watch it however, since some kids will hold it in too long and cause problems with constipation.
Even if he has never held it before, he might be holding it out of nervousness in the new setting or being daunted by the unfamiliar bathroom. (Believe it or not, I can still remember how dark the little bathroom was off the classroom, in first grade, and how unwilling I was once to pee in it because it had soapsuds in it from a recent cleaning.) If he has started to hold, then it is quite possible that he doesn't feel it when it is coming. It's a classic pattern in encopresis, in that enough holding makes the warning signs disappear.
Follow the ideas from the other posters, talk to the teacher and talk to your son about getting onto a deliberate schedule. Give him grape juice at home and not apple juice, and even give him a stool softener for a while (one of the natural ones like Benefiber). Good luck, he'll probably get by this soon enough if you are supportive and proactive.
Let me add to what specialmom has said. Sometimes the bathroom can be a scary place for kids. They don't want to use it at recess time due to all the other kids in there. If there teacher is a bit strict about letting them out during class time - it will cause problems.
So definitely talk with the teacher. Ask her for suggestions. Also, does he have any idea when it happened. If its always about the same time of the day - that can be helpful knowledge.
Hi there. I have a six year old as well. He started first grade this year and it was quite a transition for him. Once potty trained, my boy NEVER had accidents--- day or night. However, he wet himself at school one day. And that same week, several nights in a row, he wet his bed during the night. He also was having 'outbursts' that are uncharacteristic for him. I determined it was stress and the transition. Could anything like that be going on for your son? It wasn't that school was bad, he just takes it very seriously and wants to do well and it is different than what he was used to.
I talked to his teacher and him. My son needed to go ahead and ask to go to the bathroom at school AS soon as he felt the first urge. His teacher also gave the whole class gentle reminders during the day. Some kids have a hard time asking in school for some reason.
I hope he works it out----- I'm sure he doesn't WANT to go at school. good luck