If it has been sitting around for 25 years, it will most likely need an overhaul.
It costs me around $200 to get my professional clarinet overhauled every couple of years, with a student model it will cost at least $100, probably closer to $150 if it is worth overhauling. They will take all the keys off the clarinet and thoroughly clean all the tone holes in the body. They will replace all the corks and pads on the keys as well as cleaning out the rods and changing any screws that might be stripped. Then as they put the keys back on, they will oil the heads of the screws and adjusting the keys to have them work smoothly. This is quite a lengthly process, expect the repairman to take a couple of weeks to a month to finish this since he will have other repairs to tend to as well.
Like PP's mentioned, take it to at least 2 different music stores and talk to the person that runs the woodwind dept. I am guessing that the clarinet is plastic, if it is wooden, it will not work for marching band. The moisture in the air will cause the wood to expand and contract which will cause cracks and ruin the instrument. A lot of the cracks are difficult to repair and can get quite costly.
Sorry if I got too technical, hope everything works out for you.