I want to share info about United College in Santiago, Calle Rancagua 365 (Providencia) next to Parque Bustamante metro station (Linea 5) or Baquedano metro station (Linea 1).
It is a private elementary and middle school, in Spanish only. For 1st grade the price is 270 US$/month.
It is a nice school with nice people, very friendly who take good care of the children.
Also for Spanish learning only, for kids and grown ups also, I recommend Escuela Bellavista in barrio Bellavista, close to Salvador metro station (Linea 1).