How do I connect to a wifi network from the login screen on OS X Mavericks?

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.

Use a local account on your mac to connect to the WiFi, then logout again, you won't loose connection, and you'll only have to do this once since your Mac will remember the network.

I have just encountered this issue myself with a 2017 MacBook Pro. It too doesn't have any Ethernet out of the box.

It's a little bit of a catch 22 but the way to resolve this is by creating a mobile user account that will cache credentials in your user home folder. In which case you have two options.

1. The first requires you to get an Ethernet adaptor for your MacBook (Yes, I realise you don't have one of these but for completeness, I am including this as a solution because it might suit other readers) and make sure your network admin has setup a mobile user account for you.
2. Once connected via Ethernet, login that way using your network account first. (Don't worry, we only need to this for the first logon.)
3. Once you are logged on, make your user account a mobile account by going to System Preferences > Users & Groups. Click the lock icon to unlock it and enter an admin username and password. Then click the Create button next to “Mobile account,” choose the disk where you want the local copy of your network home folder, then click Create.
4. Next, you want to connect to your WiFi network. Once you're logged onto the WiFi successfully for the first time, that should be enough to get you logged into your WiFi at the login screen, without the need for an Ethernet connection every time.

Your other option is to create a .mobileconfig configuration profile for your MacBook which allows you to configure a WiFi network for your MacBook to use at the login screen. Hopefully your network admin can help you with this too if you don't have macOS server you can use. For example, using macOS Server's Profile Manager, configure a Network payload for your MAcBook for your schools WiFi network. (The SSID is case sensitive so make sure its typed correctly.) See this link for more detail on creating and editing configuration profiles.

You will may need the help of your network admin to help you correctly configure your network authentication settings if you are using Enterprise Authentication. Given the many variables and that we know zero about your schools WiFi, it's impossible to advise here a correct configuration that will work.