Teachers often do not remind small children to use the bathroom and many children are too shy to ask .Ask the teacher to remind him and make sure he goes ...
Wow, you had your kid potty trained by 1? My boys were much more stubborn as are many since the US average age for boys to be potty trained is 3.5 years. More power to you lady!!
So, why would he all of a sudden be peeing in his pants at school? Well, it is a big change. He's got a whole new routine and set up there. Exciting things are happening and leaving it to go to the bathroom is probably not desired. Or he's so engaged that he waits too long and can't make it. Also check out if he has any fears or discomfort with the bathroom there as little guys can have this issue too.
My own son when he went to FIRST grade if you can believe it wet his pants two times in school during the start of the year. He also wet at night that same week he had the wet pants at school. Very unusual for him!! But first grade was his first all day experience and he did love it but it created a little internal stress for him adjusting. So, it showed through the peeing.
So, you should tell him that as soon as he starts to feel like he has to go, that he goes THEN and does not wait. You can have the teacher remind him every 45 minutes to go which should catch it. And you can maybe make sure he gets extra sleep (we put our son to bed 15 minutes to 30 minutes early during that period of school adjustment/anxiety) and extra tlc.
good luck!