Is this sexist?

No. They make school rules based on a thought-out process that makes sense to them. There's no down side to giving guys less time. Girls have make-up considerations and it takes them a bit longer than it does most guys.

Here's a question to you: does it take you longer than 10 minutes to get cleaned up and out of there?

I have no idea what guys being nastier is true or relevant or exactly what it means.

Nasty?
Perhaps your gym teacher has a mole in the boys locker room.

Then again. Boys are nasty, pretty much.

.... and they can be just nasty in 10 minutes as in 15 minutes.

That reminds me of . . .
Well, never mind . . .

What if, however, a guy had make-up to put on? I have shamelessly done it before.

Perhaps a school gym locker room would not be the best place for boys to apply make-up. Unfortunately, not everyone understands the desire to be impeccably groomed. It's sad, don't you think?

It doesn't matter if it's 10, 15, 20, 30 minutes, etc. There is less involved regarding guys cleaning up and getting changed than girls. There is more involved regarding girls which will require more time.

I'd say the Gym teacher's remark was sexist, and someone should have a talk with her about her big mouth. Otherwise, it's no big deal. I don't buy claims that women need more time to shower and dress, make-up claims or not. Fifteen minutes is not a hell of a lot of time for that purpose, boys or girls. What kind of palatial facilities do they have that a group can accomplish all of that in fifteen minutes?

Just the reverse