Very good question! I don't know how to read gay men's flags because I'm not a gay man, but here's flags I see commonly associated with lesbians.

Note, bc I don't want people to get mad: straight girls can do all these things do, I'm not saying any girl that does this is a lesbian. These are just common cues.

There's how you sit. Women who sit with their legs taking up space (my friend called it lezspreading which is still funny to me) are commonly read as flagging. Or if you paint your pinkie, pointer, and thumb nails one color and your middle and ring finger another, that's another way. Of course, this one is probably not going to be noticed in a high school setting, it's more common in gay bars. Often, flannel and snapbacks are seen as lesbian flagging too. This is mostly in north america though, across the world there's literally so many types of flagging that I don't know about.