How do I connect to a wifi network from the login screen on OS X Mavericks? I have a late 2012 MacBook Pro running OS X 10.9 Mavericks, and it uses a network login system with my school. 

This normally wouldn't be a problem, except for the fact that I can't connect to a wifi network from the login screen. 

Is there a way I can accomplish this so that I can log in and get this thing going?

I can't use an Ethernet cable, because the MacBook doesn't have a full Ethernet port, but it requires a Thunderbolt to Ethernet adapter, which I do not possess, and neither does my school.
Reboot and hold down the OPTION button.
That should give you the option to select a wifi network.
Then select your drive and hit the arrow on the screen.