Tomorrow I learn if I got main cast in our dance school's yearly production. I'm stressing out yet at the same time oddly calm. Any tips?

if you get the part, don't brag about it.
if you don't get the part, it's okay, there are more important things and more things that can make you excited/happy.
for now, just relax.
Quote from Sing: "Sleep a good night's sleep, do a great day's work!"
Just try not to be stressed out.