I don't see it being a major issue for a single semester. If they were meant to fail Calc 1 and instead move to Calc 2, that's on them to learn the old material

I don't see it being a major issue for a single semester. If they were meant to fail Calc 1 and instead move to Calc 2, that's on them to learn the old material or they will fail. They'll be forced to relearn the material or continuously fail.