The rentrée, in France, means more than simply going back to school. Because the French take real vacations, the cities empty out in the summertime and people leave for weeks. In my neighborhood, even the small grocery stores all close and you can't reach a doctor or dentist. So you might as well leave, too. I remember how astonished I was, when I lived in L.A. after twenty years in France, to find that even the most important people were often still there, working, in the middle of August.
Then everyone tends to come back within a two-week period, late August, early September, refreshed and tan and eager to see their friends again and face the cooler days. This is why the rentrée is a big deal even for grownups.
For an explanation of what the rentrée means for French children, my fellow blogger Véronique has a good post here. And here is a photo of a little girl with her heavy cartable, which signifies that the long days of the French school system have begun again.