Firstly - you need to go into Server Admin, enable open directory, and make one server an Open Directory Master (if you don't already have one).
Secondly - you need to go into Workgroup Manager which (by default) will show you authenticated as * to the local directory. Any changes you make here are specific to the server, eg local accounts. You need to click the little world icon with the down arrow and choose the OD. Usually this will show up as '/LDAPv3/127.0.0.1', if it doesn't choose 'other' and browse to it. It will ask you to authenticate which will be the directory administrator account (not the local server account) that you were prompted to create/enter. If it doesn't prompt you, click the lock on the right-hand side to unlock it, entering authentication details when prompted. Once authenticated you will see a list of users (only two or three by default). Users created here are network users and accessible by your networked workstations. Create the required accounts/groups for your school.
Thirdly - On your workstations, log in as a local account with admin rights, open the 'directory utility', click the padlock (bottom left) and authenticate to unlock it. click the plus icon, choose to add an 'Open Directory' with the ip address or DNS name of your Open Direcory server from the first step. Then in 'System Preferences' , click 'Accounts', unlock the padlock and authenticate (if it's locked), click 'Login Options', and make sure 'Automatic Login' is disabled, and 'Display login window as' is set to 'Name and Password'. Reboot and try logging in using an account created in Step 2.