My personal opinion is to just get a micro-mixer like the Behringer XENYX 502 for around $49.95.
Major music stores have them, or you can find them online. No need to build anything and not only does it offer a headphone out to use, it also has a record out. PLUS, being a school with a tight budget, this mixer can be used for many different purposes. Easy to set up and use and should solve your pre amp situation nicely. I DJ from my laptop nowadays and I use a mixer just like this one for both my computer and MP3 player.
Just my opinion though, since all the suggestions here are also fine solutions as well.
Rock on, I'm out.