From the MinecraftEdu launcher, click the "Start Minecraft Servertool" button.
You'll then be given some options about what kind of world you'd like to create (or open).
For example "Generate a Completely Flat World" if you'd like student to build things without worrying about cutting down trees or hills.
Once you have started the server, you can see the information and change settings from this window. It also shows you the IP address that students should directly connect to in order to join your world.
Before you quit the server, remember to save the world.
I also highly recommend that you make a backup of your saved world. Copy the appropriate folder to a USB drive or a network location that is backed up. If MinecraftEdu was installed by EIPS Tech Services, saved worlds will be in C:\Program Files (x86)\minecraftedu\servertool\worlds\savedworlds
Of course if you want to try out a simpler process, you (or a student) can start a single player world and allow others to join by typing /publish in the Minecraft chat or by opening the game menu and clicking the "Open to LAN" button. Unfortunately this doesn't allow you to use many of the MinecraftEdu features.