She should be celebrating her outing with the girlfriend she's kept secret until today. I don't know why you'd be invited to her announcing that she's a lesbian.

For many people senior year of High School is when their personality solidified - At least until the next big milestone like marriage and children. So unfortunately depending on their ages and maturity don’t be too surprised to see High School like behaviors. Think of this as your turn to show you have matured and are no longer dependent on your peers.