You can boot from a liveCD or pen drive and and then use dd and netcat to copy the whole drive to another computer. Then after the system has been formatted you can reverse this.
There are quite a few posts on how to do this on Stack exchange. The OS in question is often something else than OS X, but the actual contents of the drive you are backing up do not matter. It is all raw data to dd.
However there is a not quite so minor legal matter.
A common reason why schools wipe leased student computers is because of licensing issues. Usually the school will install a lot of school related programs which you are only allowed to use as long as you are a student. When you graduate you stop being a student of that school and the license is no longer valid. The school must make an attempt to remove these programs.
It can do so by logging into your computer with an admin account, uninstalling the programs one by one, or by simply reformatting or reinstalling the computer. The latter is much quicker and if a school has to do this for all students then this is the preferred option.
It is also quite likely that you agreed to this before you got the computer (check the the small print in the agreement).
If all this is correct in your case then you can try the following:
- Check which programs are installed.
- Check which programs are legal to keep.
- Uninstall all programs for which you have no legal license after this summer.
- Go to to the IT/helpdesk/servicedesk which would normally format your drive and tell them that there is no longer any school software on the laptop. Depending on the person which helps you you may get to keep the current set-up.
I have done such work in my student time when we still handled out pentium-1 laptops with 16 MB RAM and windows 95. When a student stopped studying we were required to do such wipes. However if a student already had installed Linux and no trace was left of the original windows installation and accompanying software we did not reinstall the factory image. The goal was already accomplished. No need to bother the student by forcing yet another reinstallation.
Your mileage may vary with the expertise of the helpdesk. Some will just try to follow the rules. Other will see the intent and will try to be helpful. I was lucky enough to work in a place with relative skilled people who could think and who followed the intend of the procedures.