Both of the original two are idiomatic as far as I know. I sometimes wonder whether my use of #2 is English influence on my speech, but I notice my parents using it a lot as well, so I figure it's also acceptable natively. Still, if you need to pick one over the other then maybe #1.
Bamia, yours goes one negation too far: it reads "no longer not started", suggesting (if convolutedly) that the class has started! ما عاد and its synonyms (ما بق، ما بقا، ماعاش، بطّل...) mean "no longer" or "not anymore", while بعد and its synonyms (إسا، لسا) mean "still".