It's easy to put out the call for volunteers. It's hard to cope with the deluge of people who want to help.
Code bases are complex and/or large in any meaningful project. Even skilled programmers may take time to get up to speed. Core developers have limited time available to answer questions. (People don't scale.)
For someone working outside of a classroom, this may be fine. They aren't on a schedule and can claw their way into a community over time.
But if you plan to unleash a classroom of students on an open source project, what you really need is an instructor who operates at or near the level of a core contributor to that project. The instructor is the one who bears the overhead of scaling up developer time by funneling student questions into a high signal to noise stream.
Mozilla is a bit unusual in that they actual pay someone (David) to do that.
I know instructors have attempted to replicate this model with other large open source projects, but it hasn't been a success. Attempts I know of to work with openoffice or the Eclipse Foundation didn't really go anywhere. RedHat/Fedora might be a good place to look. Try to get in touch with Greg De Koenigsberg there.