I don’t see why. Bellyache causes stress / crying / lack of sleep, those cause headache. Completely reasonable.

And periods, which can have both symptoms, can start as early as eight years old, so there’s that, too.

Also, I don’t see why a teacher should question why a pupil is sick (and even less why she would share their private information with her partner or anyone else and still be allowed to have contact with children or any position where she might have access to personal information); parent says she’s sick, that’s all the teacher needs to know.