I agree, anything other than immediate assistance with a smile when a little one says they need to go is just ridiculous!!!
When I was picking up my son from Pre-kindergarden the other day, a boy was 'dancin' around telling the teacher that he needed to go... She hated to leave the room to take him, she was saying, "but we had a bathroom break right before nap...."
SOOOO!!!! that was before nap. And, I would bet good money that nobody insured that ALL of the kids actually went, or offered any assistance. I was right there, making a point to actively watch to see her reaction... so she asked if I could keep an eye on the few remaining nappers while she ran him to the bathroom... Poor kid didn't make it. They came back in a couple minutes later, and he was soaked. He apparantly had a hard time getting his pants undone fast enough. (Needed to be in elasticized pull-on pants!!)
Most kids are indeed naturally going to be too embarrassed to interrupt nap-time, story-time, etc... If a kid is in bad shape enough to speak up, then that usually means they GOTTA go!
Plus, there are big physical considerations, especially with pooping... My son had major problems when he was in Mothers Day Out because he was 'holding-it'. He ended up with the all too common toddler bowel problems, resulting in a trip to a specialist!
Teachers definately need to be really proactive when it comes to this! And, encourage and assist, not add restrictions, embarrasment, time-out, etc.!!!!
Maybe you could print off some of these messages and take copies to the principal and the teachers.