"I skipped class" is 授業をサボった in Japanese.

I'm not sure I understand your question. "I could skip class" is good English. It means that there is a POSSIBILITY of skipping class in the future. I might skip it or I might not skip it. "Can you meet me tomorrow at 2 p.m.?" "I'm supposed to be in my physics class then, but if it's important I could skip class." "I skipped class" is good English. I means that you actually have already skipped a class. "Yesterday, it was important for me to meet Elsa at 2 p.m. so I skipped class."