Nests being destroyed

I have a children's blind school at the back of me with lots of trees and greenery. I heard squawking early hours and woke to find the 4th nest destroyed on the flags with smashed eggs and a lot of mess. Really upsetting to see. We do like everywhere else have lots of magpies. Im guessing they are the culprits! What can I do to prevent or keep them away from the nests? 

Thanks Jo