Boys have to wear tight fitting shorts, it stops them bagging out with air when they get into the pool. Plus leg movement is easier to see.
Girls have to wear the one piece because it is a swimming lesson, not a fashion show and part of the curriculum is to teach children to swim.
I go with the swimming group in school, so I can tell you first hand that the one piece is quicker to get off, no faffing with two lots at changing time (which is 10 minutes from getting out of the pool, shower, dried, changed and pack all your stuff away, coat on and line up, make it outside and back onto the bus with irate driver)
Children often forget the other half of their bikini or tankini and it does ride up in the pool.
Personally I feel it sexualises children and I hate a bikini on a child, tankini is fine but not for swimming at school.
While we are at it, if you child wears either a swimming cap or goggles can you please teach them to put them on
and if they have long hair, plait it for me as trying to wind a long ponytail up and put a cap on for a girl is hard for me as I have two boys
We take 60 children at a time. 180 in total in a morning.
Oh, and teach them to brush their hair from the bottom and they do not need deodorant/moisturiser/body butter/masses of hair product. You would be amazed at what they bring in, did I mention 10 minutes to get ready? 