I found this big C rail near my school on the edge of a sloping bank, and I'm wondering if my skis will slide on it if I round down the edges with a grinder
no, aluminum is a bitch, you might still manage but i don't know, form what i remember its really sticky.get old skis and pull out the edges,, best way, safest way
As a rule of thumb, two different metals hardly ever slide well together. This counts particularly if those metals have great differences in hardness, like steel and aluminum do.
aluminum is too soft of a metal. best is to get steel and hit it with a grinder to smooth it out and polish it. slap some paint on it periodically to protect it from rust and then wax it.
You would struggle to slide well on aluminium if thats what the rail is made from, aluminium is actually a lot softer than the steel edges on your skis, this causes the edge to dig into the rail.