it has been a while since your post so you may have found the answer somewhere already.
To do this investigation scientifically you'll need samples of different metals, preferable the same size or weight. Remember that rusting is oxidation and some may not be susceptible to it. Weigh them carefully with a sensitive scale (school may have one or nearby college chem lab). Then expose them under similar conditions to salt water (say, 1 T/cup water - but better to use metric measures!). Wait, say, 24 hrs, weigh again, another 24, etc. for, say 3-4 days. The metal that rusts takes on O2 from the air and should weigh more. Counter intuitive, no?