What movie is it where these guys in santa masks kidnap these kids?

its kinda old, but not in black and white, prob 70's. it has a teacher taking a whole bunch of her students of all different ages out on a trip but these guys in santa masks kidnap them. at one point they meet this old couple then they swim through some water or something. anyways they end up making makeshift weapons and killing them viciously. anybody know what i'm talking about