1 Lucas: There’s something very weird about everything going on around here at the moment.
2 Mike: You mean a strange kid showing up mysteriously with no parents?
3 Mike: And other kids disappearing and nobody knowing what’s going on?
4 Lucas: No. I mean it happening while school is in. This sort of eerie kid adventure stuff usually only happens over summer vacation.

Although Stranger Things is set in the 80s and is clearly an homage to all those wonderful 1980s "kids having adventures" movies, it is a little unusual in showing the kids actually going to school.

E.T. the Extra-Terrestrial did have a scene with Elliott at school, but I'm struggling to remember much school in any of the others, which basically all seemed to take place during vacation time.