How do ballet dancers stay fresh throughout hours of classes?

Professional ballet dancers are finished with school.

They shave and powder their bits and pieces.  You freak.

Usually - the way to limber up is all of those demi-piles, and the rond de jambes all at the barre. To further limber up, also at the barre they do the ballet jumps like jumping jacks with reps, like soubresaut, temps leve, entrechat deux, entrechat trois, sissonne ouverte/fermee, and the glissade in place alternating feet, usually either at the barre or au centre (in the center, without barre support). 

They also are thinking a strong French mindset throughout the classes, knowing the moves will be in French, and it doesn't matter if they are taught in the Cechetti way or the Russian way of ballet. 

Well, mostly they don't. But they shower or take a bath every day and put on clean clothes, so even if they do get a bit sweaty, it's only one day's worth. Yes, you can smell the sweat, but it's not exactly "bad".

Deoderant and i’m not sure about everyone else but when i was rehearsing a pas de deux when i was in high school my partner didn’t sweat.  She was soooo thin and i don’t think she had sweat at all and we rehearsed for 3 hours a couple days a week for the school dance recital

Ha. Why do your think they smell good? They are all dancing, sweating, smelly. It really doesn’t make a difference when everyone is is at the same level of odor. 

They think of their boyfriends and or future boy friends.

Years ago, I had a girlfriend who was a ballet dancer.  Believe me, they don't!