Praise be to Allah.
What they are doing is not permissible, firstly because it is an obvious lie by which they are claiming to have an excuse. Secondly, they are failing to pray, whether they are not praying at all, or they are delaying the prayer until after the proper time, or failing to pray with the congregation of women. Thirdly, they are praying after that, at the time when they really do have their period. You have to advise them, remind them, exhort them, and explain to them the sin of lying and the punishment for delaying the prayer beyond the proper time, because Allaah says (interpretation of the meaning):
“So woe unto those performers of Salaah (prayers) (hypocrites),
Those who delay their Salaah (prayer from their stated fixed times)”
Any girl who is found out to have delayed her prayer and prayed when she was menstruating should be punished in a manner that will deter her and make her give up these actions which are denounced by Islam. And Allaah knows best.