UNDERWEAR & AESTHETICS
Dance teachers ask dance students to not wear underwear or panties under leotards because dancing is all about aesthetics. Underwear that can be seen distracts from the aesthetics of the movement being created.
Another reason is because underwear can be distracting to the dancer. Leotards and tights are tight and this can lead to uncomfortable underwear bunching up the crotch, or if the underwear is old it can start to slide pulling down on leotards or tights. If the underwear is too tight, or pulled up to high the wearer can constantly be trying to pick an uncomfortable wedgie from their bum.
A lot of dancers find it most convenient and comfortable to wear a tampon, but some girls, especially younger girls, opt to wear a thin pad. Wearing a pad is perfectly fine, just make sure to choose one with a high absorbency level. If needed in this case only, you can wear flesh-colored dance underwear. However the underwear should never be visible. We advise Adela underwear by Wear Moi
Information taken from Danceparent101