My 3 year old keeps having potty accidents at school - holding her pants and crying - but not telling the teacher she needs to potty - what should we do?

If she is using the potty at home, then it's obvious she is potty trained, so it seems there is something about being at school that's making her either uncomfortable or afraid to let the teacher she needs to go to the bathroom.

Have you tried talking to her teacher to see if she would be willing to offer to take the child to the bathroom throughout the day? You were the one who potty trained her, so maybe she's not comfortable in telling someone else that she needs to use the bathroom.

Have you looked at the rest room facilities at the school? There may be something about it that scares her, or makes her uncomfortable. Maybe the toilets are larger or taller than yours at home, which could make her afraid of falling in. If it's one with several stalls, that may be scaring her, confusing her, or making her uncomfortable.

If it were my child, I would spend a while at school with her (explain to her teacher first), and take her to the rest room myself. I would then encourage her to use the potty while I was there with her, partly to see her reaction, and partly to reassure her that there is nothing to be afraid of, nothing to be uncomfortable about in using the rest room there.

You could also try letting her teacher take her to the rest room while you are there, to assure her that it's completely OK for the teacher to take her.