School Work or Schoolwork?

I work as an English teacher and have been taught the correct way of writing it is Schoolwork. However, I worked as a substitute teacher and children told me their teacher taught them to write School Work and now I'm quite confused. Let me explain this: sometimes when we do an exercise and children have to put a title in their notebooks we, English teachers in my country, tell them to write Schoolwork (or School Work) as a title. Which one is correct?