How do you hide being a lesbian?

I’m an 18 year old female still attending high school. I have an under the table type relationship with another girl at my school. I really don’t want my family or friends to find this out, and I just have no idea what to do. My sister knows I never have boyfriends, but I have hooked up with guys. I’m not sure if I’m lesbian or bisexual, but that’s a different story. My sister can find out anything she wants at school. So it’s really difficult lying to her in order to hang out with the girl I like. All my friends think I am but don’t say anything to me. My family has questioned me as well. I just really prefer not to tell them. I talked to the girl I like and mentioned what if I hooked up with a guy so the rumor of that could spread around the school and kill all the thought of me and her being together. She obviously hated the idea. I just need some guidance with this. Should I ignore it until I go to college? Thank you for your time to answer this.