I remember throwing plastic bottles off a river bridge with my class in 3rd grade. While it's true that the river connected to an ocean some 400km downstream, I doubt they made it further than the next best river bank.
I sincerely hope that since then schools have stopped encouraging children to throw plastic in waterways like this.
In the 1970s when I was in grade school we attached postcards to helium balloons to see how far they could get (some people got responses that indicated they traveled for hundreds of miles). That probably would be even less acceptable today in terms of littering as deflated balloons often get eaten by wildlife to ill effect.