best way to connect remote branch to main school network

[bicky]

Hi our school has just taken a charity partnership with a small learning center 5 miles away. Over there we have installed 3 PCs networked to BT ADSL router. When our Staff goes there they RDP to our server and login with their regular user accounts.

 

We are looking to expand our network over there so that those PCs will be a part of our network; what are the cheap options available. Because its a charity partnership, we dont have much budget.

 

We user ISA server as the firewall in our main school.

 

Any help would be much appreciated, thank you.