NO ONE has the right to check UNDER anyone’s garments. THAT in itself is NOT TZNIUS and anyone who thinks it is should have their heads examined. If the over garment covers everything the undergarment does not have to be closed up. It should be comfortable. That is going way too far and I could absolutely understand why kids would be turned off by it.
I mentored a young lady who lost her mother to a brain aneurysm. She was very young only 11 and she was one of 5 children. Her school did not know how to deal with her although she was a typical BY kid. One of the things she told me that hurt her the most was that they kept pointing to her top button and asking her to close it. She said they point to her button which was just three inches from her heart. They were more concerned with her opened button than her broken heart. That caused her enormous pain, so much that she did not know how to deal with it. It compounded the pain she already did not know how to deal with concerning the loss of her mother which her mechanchim also did not help her with. That caused her to rebel.