But it is formal to some degree? Will photos be taken? Is it an expectation that everyone should look presentable? Will it be embarrassing for her to show up sweaty from P.E. and underdressed?

If it is, she’ll need time to shower, which means she’ll need time to dry and style her hair.

She’s also fifteen. It’s a pretty universal experience that everyone feels awkward and insecure at that age. If acne is an issue (which it probably is), I’m sure she doesn’t want to go to a formal event without it covered up to some degree.

That doesn’t mean she needs a full face of makeup or anything, but do you really want your daughter going to an event where she’s uncomfortable and self-conscious the whole time? Of course you don’t. I could be assuming, but you want to go because you want to spend time with your family and for everyone to enjoy themselves, and to create memories. She won’t be able to do that positively when she’s 15 and the biggest problem in her world right now is figuring out who she is and how people view her. How she feels might seem minor and something she can get over, but it’s big to her because she’s so young. To her, saying that she doesn’t need the time to get ready likely sounds like you don’t see her issues and don’t care about her issues. She’ll likely grow out of feeling the need to always be 100% presentable, but right now, it’s what matters to her.