User blog comment:B214/Kuoh Academy students finding out that Issei stays with the ORC girls and is dating Rias./@comment-24098614-20131215011722/@comment-16281343-20131215071945

Maybe the girls do know but the boys don't. Maybe its an unspoken rule for the girls not to talk about Gasper being a boy to preserve whats left of the sanity or mentality of the boys.

Issei being smothered with love by beautiful girls is already too much for the boys to handle. If the thing about Gasper being a boy is revealed to them, I don't think they can recover from the shock.