In this case, I made two statements to the client:
1) Professionally, I did not believe the work was either original or non-obvious, and that for me to sign a declaration otherwise would be perjury.
2) Their agreements with me did not allow for them to patent my work (something for which I read contracts very carefully and seek professional advice on as needed).
They backed down pretty quickly at that point.
You might have luck with a similar approach: If there's something legally improper about the inventorship, or your school does not actually have the legal ability to compel you to patent your work, you may be able to get them to back down in a hurry by laying out the facts of the matter.
But I am not a lawyer, and if the school persists in this behavior, you'll want to speak with a good patent lawyer. You may want to call a couple of patent lawyers and describe the situation to them. You can usually learn a little bit about the merits of your case before they start charging. :-)