Chatting to folk about this today and it was mentioned that the school at Welshpool does very well. This isn't a high population density area either really, is it? Nearest big town is Shrewsbury to the East.
If its true about Welshpool and Shobdon etc. then there must be something more to it. Wouldn't say either of these were particularly wealthy but could be wrong! Possibly it is the business folk involved, as already suggested.