Single Gender Swim Classes and Swim Times

[SwimForHealth]

My nephews attend a school where the swim classes are coed. They are somewhat shy wearing swim suits around girls. About 3/4 of the school is female. They have been allowed to wear rashtops with their suits.

Should swim class be mixed or single gender?

Similarly the open swim times in the school are co-ed and girls only. Should male swimmers have single gender swim times as well?