Let's just say Kerr doesn't correct anyone. Kara knows she's a girl. I figure everyone else just assumes. She doesn't use the locker rooms at school and tries to use the bathrooms that no one else does.
The teachers probably know she's a girl, but is questioning her gender identity- so because the school wants to be 'woke', most teachers don't use gender pronouns with her (her Aunt may be a little bit overprotective in this)