I know when I went to prom (2008) both a person and their date had to sign a little contract attached to the ticket order form, have their parents sign it, and specify who you wanted at your table. Any date 21 or older had to be personally approved by the principal and at my cousins' high school (same city), Freshmen had to be approved as well. So depending on the school's rules, she may have had to tell administrators.
That said...what. the. fuck. At the school dances at my school, single girls would often dance (yup, freak dancing and all) together to look hot or attract dudes or just to get on the damned floor. Since at my prom (yup - the best kind of evidence, anecdotal!) no one really was kissing heavily or making out...what the hell is the difference between that and lesbians?