Another reason is the cost of supplies. Education is "free" but all that means is that you don't have to pay to have your kid walk through the door. But they don't provide any supplies, including textbooks, paper, pencils, etc.
When we go to Mexico I throw bring along a few school-related things like writing tablets and notebooks. When I meet the housekeeper I make it a point to ask about family. If she has school-age children I leave some of that behind for them. sometimes if we get to Wal-Mart I'll load a gift card with about 100 pesos and leave it with our trip with a note that it's for school supplies. (Of course I can't guarantee that's what it's used for, but I figure that at least the thought will count.)
If we're out and about and we go buy a candy store, I'll also pick up some dulces for the children as well.
All of that, of course, is on top of the normal housekeeping tip.