So... You didn't buy tickets and you didn't ask a girl to go with you, and then you told her after the fact through a facebook message? Yeah, you done goofed, son.
Luckily for you, from what I remember of girls back in high school, just go on a walk with them and let them vent all their frustrations out on you. It might take an hour and a half or so, but things'll be right as rain, and you may even get to make out after all that if you look sorry enough.
Or you can just take comfort in the fact that none of this will matter after you graduate and she's nothing but a "What a wild four years! Have a great summer!" in your yearbook.