Not teaching that communism is evil in a social studies class is like teaching evolution is not real in a biology class.
There's a difference between fact and opinion. Fascism and communism are not inherently "evil", I happen to strongly disagree with both ideologies but they are just that, ideologies. Teaching that certain beliefs are uncritically evil or wrong is indoctrination. In Eastern Bloc classes they were taught capitalism is evil, America is evil, so why is teaching that communism is evil any better than the Soviets?
It's better to present the facts, talk about Stalin, Mao, Hitler, and Mussolini and not just telling them to accept one belief.
Indeed, history class should not be taking its time to moralize. Simply lay out the historical records of fascism and communism and it should be clear to everyone why these are horrific ideologies. If anything, explaining at length why they are terrible will probably only prompt teenagers to identify with them in an effort to be as edgy as possible.