Period-related absence is not truancy
Schools should not penalise period-related absence for pain or heavy bleeding. If she's regularly missing school, that's a medical signal, not a discipline issue — and worth flagging to both the GP and the school nurse or head of year.
What to ask the school for
• A no-questions toilet pass during her period.
• Access to free period products (state schools in England, Scotland and Wales are funded to provide these).
• Permission to carry pain relief if age-appropriate, with a form on file if needed.
• A named adult she can go to — school nurse, pastoral lead, or head of year.
If she's being missed
A short email to the head of year is enough: "[Name] is being investigated for heavy/painful periods. Please can we agree a plan for missed lessons and toilet access?" You do not need a diagnosis to ask for reasonable adjustments.