It's not clear whether the children in question are in fact sharing a bedroom, or sleeping in separate bedrooms in the same girls' boarding house. Obviously both of these scenarios raise issues, but perhaps not quite the same ones.
If you're concerned by this, you should be aware that schools in many areas may be following guidance that tells them that they should offer mixed-sex sleeping accommodation on school trips if that's what trans children say they want. There haven't been very many school residential trips for the last couple of years, for obvious reasons, so schools are only now having to deal with the practical challenges of this guidance.
If you have school age children I strongly encourage you to contact your child's school now and find out what their approach is going to be. Don't wait until you have to deal with a specific scenario. These articles might offer you a useful reason to raise the issue now.
This approach raises significant safeguarding risks for all children, including trans children. @Datun I think trans boys saying they want to share a bedroom with boys is absolutely a foreseeable scenario, and it's very worrying to me that schools/parents will continue to be too scared to say 'No, this is not appropriate'.