12:03 p.m. on August 31, 2006 (EDT)
Some great suggestions here, especially Ed's hot water bottle. I use that on my winter treks and high altitude expeditions to prewarm my -40 deg bag.
Another suggestion that is less expensive is to get one of the fleece "sleeping bags" from Walmart/Target/Big 5/etc and use it as a liner. Even a fleece blanket formed into a liner with safety pins works. This will give an extra 10-15 degrees.
Sleeping in dry long johns or a dry parka will warm things up. Just make sure any clothes you sleep in are dry (not damp and sweaty from the day's activities) and you remove all the hard lumpy things from your pockets.
A really cheap way to get an overbag is to borrow a summer-weight bag from family or friends and use that as an overbag (gee, golly, guess what an "overbag" really is? - some companies make a 2-layer bag in which the outer overbag is a summer 50 deg bag, the inner is a 20 deg bag, and the combination is a -10 to -20 deg bag).
As already noted, there are synthetic bags that go down to -40 deg. The Dark Star is one. They are much cheaper than down bags of the same rating and are non-allergenic. I have a North Face Bigfoot, which is a -10 deg bag (hmmm, I never use it anymore, and it's in good condition. Maybe I should sell it to you. I only paid $100 on a promotion that the local TNF store had).
The 2 big problems (figuratively and literally) for most synth bags is that they are heavier than down for the same insulation value and they are bulky, which shows up as being hard to compress into a stuff sack. These have both been changing in the past 3 or 4 years. The best current synth is Primaloft. It comes very close to down in both insulation for weight and stuffability. I would place it as equivalent to 500-fill down (these days you can commonly get 700 to 800 fill in top quality bags). Polargard's 2 best varieties are close, and a bit cheaper than Primaloft. The lifetime of synth is not as long as down, about 10 years for the synth in my experience and up to 40 years for down (I sometimes use my 45 year old Karakoram still).
If you are inclined to buy a new winter bag, take a look at the Integral Designs website at their Primaloft bags. They are premium bags, a bit on the pricey side for synth, but well worth it. Marmot also has some good synthetic bags. But keep in mind - a bag good to -20 or colder is very specialized. You won't want to use it in summer at all, only late fall, early spring, or during the winter.
"Keep in mind that I am a high school kid and I am on a budget. "
Hmmmm, sounds to me like you are a pretty mature and thoughtful "high school kid". I wish that even half of the "high school kids" I have to deal with put as much into it as you have been in your posts and excellent questions here.