Hello, this are some words used in a classroom: chiquilines (Kids), hagan silencio (be quiet), abran los cuadernos (open your notebooks), estudien la pagina --- del libro (study page --- on the book), este es su deber de casa (this is your homework), sean amables (be nice), por favor copien esto del pizarron (copy that from the whiteboard, please), presten atención (pay attention). If you are talking about the students' work you can say: muy bien (very well), tenes que estudiar más (you have to work harder), hacelo de vuelta (do it again). Hope I have helped you :)