Im really scared..i don't kow what to do anymore?

so i made a huge mistake this year, i flrted with a guy who has a girlfriend, and (i didn't know he had one)becasue we are in different schools,i try apologizing to her(she jsut graduated) but she says the whole school knows what happened,and i don't wanna enter highschool like this.how can i fix it,i've tried talking to her but it won't work,what can i do?