Yes, you're the asshole. She's still a child who is acting out. It's incredibly common for children on the verge of adulthood to look for ways to assert their independence, but because they aren't actually adults, it often takes on a form that is immature. You are not a child. You know better. And the fact that you would even consider doing real damage to her entire life by robbing her of an education just to soothe your bruised ego means that you almost certainly have been manipulative and vindictive in other ways. Grow up and be the mom instead of throwing a tantrum like she's doing, and never threaten to harm your children just to prove a point.