A lot of kids walk down our road during school term time as there are 5 schools within walking distance of our house, and if I had a pound for every wrapper we and our neighbours have had thrown in our front gardens we could hire a private security service.
Here's what to do about it (if you are really mad about it):
time there's a wrapper in your garden you call ALL of the schools and rant for no less than ten minutes about how they aren't doing their jobs if that's what they teach the children and whatever else you can think of (don't cuss but talk in upper case
and don't let them get a word in). It would be good if you could also get your neighbours to do this.
Shortly after this kind of campaign you'll see the schools beginning to take the problem serious, either by employing someone for the sole purpose of listening to your rants or by trying to teach the kids not to litter (not that it is the schools job to teach kids this sort of thing, but they are the ones you can rant to... they'll then have to contact the parents of the little delinquents).