没 + (v) = did not + (v) - the verb is a completed action
不 + (v) = do not + (v) - the verb can be in any tense.
You can say 明天不来上课 (will not come to class tomorrow) but not 明天没来上课 (did not come to class tomorrow)
You can say 昨天没来上课 (did not come to class yesterday) or 昨天不来上课 (did not come to class yesterday)
You can say 今天没来上课 (did not come to class today) or 今天不来上课 (do not come to class today)
他又没来上课了 = he did not come to class again (没 indicates the absence of)
他又不来上课了 = he does not come to class again (不 indicates refuse to)
他又没来上课,又没通知學校 = He didn’t come to class and didn’t notify the school (没 indicates the absence of)
他又不来上课,又不通知學校 = He doesn't come to class, and he doesn't notify the school (proactively refuse to do either)