Pass. That's one of the questions we'd need to answer. Certainly some method of securing the link between AD and filter is required - though amazon I believe offer a VPN. Wether the present (or next-gen (SOON!)) auth daemon would perform well over latent links isnt something we have tried - so it may need an alternate method, or maybe some fiddling with the auth cache. Again, this is on our "to play with" list, and we would welcome your input.
Does the Active Directory server also have to be cloud based, or does that run on servers inside the school somewhere?