Yes, we always use a positive tag question after a negative sentence:
- You shouldn't take this medicine, should you?
But we don't always use a negative tag question after a positive sentence:
1A: [So,] I am impatient, am I?
would be used when you at last meet the chap who's been telling all your workmates that he thinks you're impatient. It's a request for clarification of his view, or even confrontational.
1B: I am impatient, aren't I?
uses the usual tag question, here begrudgingly asking for confirmation (which one hopes will be given in a not-too-unpleasant way) of one's self-assessment.
2A: The class was dismissed, was it?
This can be used in a way showing surprise at hearing the news, or in a challenging way (challenging the decision to dismiss or the statement that it had been dismissed) as in 1A. It could also be an unmarked form, equivalent to 'Can you confirm that the class was dismissed?'
2B: The class was dismissed, wasn't it?
This is not unmarked, but conveys (more than 2A) the questioner's belief that the class probably had been dismissed, or the questioner's view that the class should have been dismissed (emphasis on was).