Marketing in the gatekeeper model pitches the product both to children and their parents, it suggests that parents (or schools in loco parentis) are right to exhibit veto power in regards to what children consume. School districts really have no other option - it's potential political dynamite if a nine year old girl nags Mom for Revlon nail polish based upon advertizing delivered while using the schools computers for an academic project.

I'm not saying that Microsoft is motivated purely by altruism - however, Microsoft could probably offer everybody (not just students) ad free search and still be massively profitable and there's not a damn thing Google can do about it.

Privacy issues are a potential long term limitation on Google's business model and kids grow up fast.