They could be made out of cupronickel (like the nickels from 1982 - 2000) which are also non-magnetic. You can do a few easy tests to see if it is silver:
1) ring test, drop the medal onto the table, if it is silver it will have a higher tone to the ring (compare it to a toonie or loonie)
2) weight, Silver will be significantly heavier than a cupronickel coin, or zinc/copper coin.