>28 x wireless keyboards and mice��

You��definitely��should not use so many��wireless mice in the same room. keyboards are ok but the interference and drops on the mice are frustrating. We used them in a room of about 16 poeple and we had to convert to wired. Also you might want to check if r pis bluetooth has drivers yet.

>For each row have a wireless ethernet router��

pretty sure a hub/switch is cheaper, you can get 10/100 for under $20

Kind Regards
Rowan Schischka