Just bought a 97 GT

Gears!

First of all, make sure your priority is school. Even if you have to quit working and take out loans, do it. The repayment for having a degree is huge (but choose a useful degree like business, engineering, ect).
Next, you just got the car, so do the regular maintanence items. You don't want to spend all your money on mods, then realize the car needs a transmission and it will cost you 2 grand... and pay +15% on credit card interest. So consider changing the trans fluid, radiator fluids, plugs, shocks/struts, fuel filter, ect. You will be amazed how much nicer the cars feels just from doing basic maintanence and you will ensure that you have a reliable car to get you through the next few years.
Lastly, mods. There are some really cheap mods you can do if you have some mechanical ability and shop carefully. I've spent around 400$ total, and gained probably 30-40hp on my GT. Here is my summary:
O/R H-pipe - just under 200$. I also made my own MIL eliminators to prevent the CEL.
PI intake - Found a nice PI intake (with aluminum crossover) from a police interceptor at the j-yard for 15$. Add in gaskets and fluids, and it cost me something under 60$.
PI cams - biggest pain in the arse to install (I pulled the front cover to replace the chain guides at the same time). I bought new PI cams on ebay for under 40$ shipped.
underdrive pullies - again, a lucky j-yard find for under 10$. marginally worth it though.
K&N filter - 60$? I don't remember, but you have the CAI already.

Beyond that, a tune helps. I bought a tweecer to tune it myself. Then, you could consider PI heads to increase airflow and compression ratio.

+1 on school !
If it only has 120,000 miles then you're looking pretty good. If tires and brakes are good then exhaust is the most fun/noticable mod you can do on a budget. Not to mention exhaust is fun once you get it installed, LOL.
Tint the windows, wax it up, and have fun.