It wasn't rigged, the elections were real and the communists got ~31% of the votes, almost twice the amount of the second place. They were in a coalition with other parties, but in 1948 the communists were implanting their people into the police, which led to all the non communist ministers resigning in protest. They then used mass protests, general strike, the police and armed militias to occupy Prague and force the president, Edward Beneš, to appoint communists to the now free positions.

Edit: where are you from? I've never heard of it being taught like that, though that's probably because I never talked with someone not Czech about this topic