I've never had to enforce this rule in a high school game, just middle school. (Our board has made it clear: this rule applies at all levels.) Most people will not raise a fuss when you enforce the undershirt rule. If anything, they get a bit annoyed of the lack of consistency, and that they weren't informed before about the rule.
The one exception: Middle school girls. I saw one girl burst into tears when she found out she couldn't wear her T-shirt, one try to tuck the sleeves into the uniform (doesn't work), and one outright refuse to play without a t-shirt. (All cases: red unis, white T-shirts.) That age and gender make for a sensitive combination.
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