There's such a striking difference between an empty classroom and that same room when it's full of children. I don't have to tell you that the energy level drops considerably once the kids have walked out the door.
I'm an introvert, in the sense that I draw my energy from time spent alone. So my days at school are an aggregate of our contrasting energy levels, held together by shared enthusiasm. It's exhausting and exciting, and I am so blessed to be able to make a living doing something I love.