You have (at least) two options.
If you don't mind imaging one Mac at a time, you can install OS X on an external drive, boot from it, and use Disk Utility (Applications>Utilities) to create an image of the source drive of the "model" machine. Then connect the external drive to each machine in turn, boot from the external drive, and use Disk utility to "restore" the image to the target (internal) drive on the machine.
A more complex, but more efficient, method would be to set up a
NetBoot server (on any machine running 10.5,) and use
NetRestore to deploy the image across your network.
Admittedly, using NetBoot/NetRestore is a more complicated way to image the machines in your lab, but in the long run, it will make imaging them easier. (I use this method in a lab I manage.)