Because a teacher is a role model and if your teacher is a role model for religion in a place that should be the one place free from it, then that's not working.
School can only be a safe place for children to take a breath from a religious background if religion stays completely out of it.
You think it can be a safe space when the girl gets told by its parents "look the teacher is wearing this, so you need too" even if the teacher isn't saying it?

Unfortunately religious clothing isn't just clothing like every other and religions of this world (not just one specific) are not about safe spaces, they want to be everywhere and they want to occupy school too. A secular country can not allow that.

You want uniforms?

Problem is, the "unless" part is different for everyone. Lot of people find religious stuff offensive, while the revealing stuff not.