Can I say "It wasn't a single time/occasion/action when she decided not to attend classes"? Would it be proper for written American English?

Answer 1

Change "it wasn't" to "there wasn't" and you're good

Answer 2

Indeed: "There was not a single time when she decided not to attend a class" In Britain I might write "There was not a single occasion on which she missed a class". Certainly "action" is wrong in that context.