Okay, no elementary school in their right mind would ever do this. I would never recommend that anyone do this.
First off, the school gets a portion of the money of the school portraits that they sell, so they have a vested interest in making sure that the school kids buy their pictures from the school photographer- not you. It would be money out of their pockets.
Second, a school is not a place where people get free advertising for their business. If you want to advertise, take out an ad in the school newspaper, church bulletins or other parent-related circulars.
To me, a photographer offering free sessions to elementary school kids flies the flag up the 'pedophile-warning' flag post. That's a list you don't want to be on.