No lesson changeovers as I work in primary.
8.50-10.50 lessons
10.50-11.10 playtime
11.10-12.15 lessons
12.15-1.15 lunchtime
1.15-3.20 lessons
3.20 hometime
So you could be expecting a child to go over two hours without going to the toilet. Especially first period as they may have had a walk to school without an opportunity to go. They might be Y6 but they are still kids.
You also want every girl on her period to bring in a note from their parents? What justification is there for that invasion of privacy, especially as you are dealing with girls that may just be starting their period?