|Interesting issue. The best solution is probably with cable :-). \|
Other solution is to make tunnels between buildings. This can be done with open vpn, or with some routers. For me the best free router is vyatta (which i am using). Just download install on a computer (vyatta requirements are minimal - it is linux after all :-) ), and set the tunnels between buildings. On vyatta site has a great documentation, so it will be easy for you to setup.
The other way probably would be with RODCs in each building which for me is not the best solution :-)