My sister got harassed at school for having a girlfriend.

My sister is fifteen and came out to me today as bisexual. She said she's been seeing a girl at school, but it's been a secret thing because she knows how other kids would react. So she said they kissed in the bathroom (a little kiss, she said they weren't making out or doing anything inappropriate). Another girl walked in, started screaming "SINNERS! SINNERS!" and ran and told a teacher. Now there is a sign on the girls bathroom door stating that the doors will now be locked and if someone needs to use it, they have to get a key from a teacher and may only go one at a time. Am I wrong for being offended by this? That's my baby sister. She's bisexual, she's not going to fuck every girl she goes to the bathroom with. The only reason they kissed in the bathroom is because they were too afraid of how people would react (girls and guys are allowed to kiss at her high school as long as it's just a peck, which is what this was). To top it off, she said boys were yelling at her in the parking lot calling her a lesbian. I'm very angry. I don't know what to do. This kind of shit has got to stop. I must protect her at all costs and I'm really fighting the urge to take down some fucking teenagers and the school board. I just want a freaking world where my sister can kiss whoever she wants.