What kind of school are you going to build? Customize the whole school and make sure it fits your style. Change the wallpapers and set the type of the floor. Will your students love the carpet or will they hate it? Start with simple classrooms and upgrade them to the best ones in the world. Wooden tables or high tech school boards, so many options, but you are the one who has to make the decision. To make sure your school won't become a dirty place, build bins. To make sure your teachers are happy, decorate the classrooms!
Cool decoration inside is nice, but planting trees and flowers outside your school gives it a whole other atmosphere. Place some trees on the outside, build a flower field. Make it cozy. Or create a prison. As long as you feel the school belongs to you.