Also a teacher.
If I let every child who asked to go to the loo out, I would have half a class most of the time.
I get it, I honestly do. I started at 11 and my "cycle" was two weeks of ridiculously heavy (frequently bleeding through night pads in a couple of hours) to two weeks off for about a year.
Teachers are, in almost every school, not allowed to let children leave to go to the toilet unless they have a medical pass. If ir isn't this way you end up with vandalism, truancy, bullying.
Nonetheless, teachers use their discretion and as a previous poster says, if the pupil is visibly distressed, completing work extremely quickly or bargaining their time, you know they're genuine.
I get asked by pupils to go to the toilet roughly 7 times per lesson. And their are girls who will lie about their periods (I know this because one girl in my class was found by their pastoral care teacher to have been lying to get out of class 7 times a day for 3 weeks).
Its unfortunate but with forward planning 90% of accidents can be avoided. If you are extremely concerned for your daughter I would advise you to contact their pastoral care teacher and discuss the fact that your daughter has extremely heavy periods. This will allow their pc teacher to let her classtoom teachers know that they should allow her very occasional trips to the loo.
Also really shocked to name-calling here. I get that it's an emotive issue but teachers are certainly not ignorant of how to do their own jobs. Would you say that to someone's face?