What is a tidy slip?

[Pyjamarama]

I always wondered ever since I was a child. Is it picking up litter or something?

In Back to the Future, Mr Strickland the teacher gives Marty a tidy slip.

What is it?

 

[UpUpAndAway]

Are you sure he didn’t say “tardy slip”?

Note that I haven’t seen the film, but from your description it would seem to fit the situation.

Tardy slip (Wiktionary definition)

 

[Methodist]

My first thought was exactly what @UpUpAndAway says: I think you are mishearing this and it is a “tardy slip”, i.e. a piece of paper noting that somebody is late for something, generally a child being late for school.