Ok, so just go into the Panel > Domains > registration. You should be able to click modify across from your domain - and just point the name servers to your schools.
If you can't just point your domain to their name servers, I'm not sure if you can add a cloaked or mirrored domain with no hosting account or not. I don't see anything off hand on the Dreamhost website about what services they offer with domain registration.
You may want to contact support and see what your options are if you can't set the name servers to forward on to your school.