This is massive safeguarding red-flag. Not only is it neglecting the kids' mental and emotional health, it's also putting them at risk of humiliation and child-on-child sexual harassment.
See screenshot for how it's described in the Department for Education's 'Keeping Children Safe In Education' (or KCSIE, the bible of safeguarding in Education, every member of staff and governor at school should be familiar with it).
When you write to school, state explicitly that you're raising a safeguarding concern (quote from KCSIE), and request it's investigated by the Designated Safeguarding Lead (usually the head teacher), as per the school's safeguarding policy. This will make them take notice rather than fob you off, they'll have a duty to investigate. Also say that your child will not be changing at school with immediate effect, and ask if school would prefer her to go to school in PE kit or be withdrawn from PE (make it their choice). If you're not satisfied with the response to their investigation, escalate to the Chair of Governors.
If you can get a few parents to raise similar concerns, as a co-ordinated effort, even better.
This kind of thing makes me so mad. Apart from the whole safeguarding issue, why on earth do they want to waste precious school time for kids to get changed then changed back? It's just so stupid!