In this instance, it would probably be helpful to you to say so - especially given what your doctor thinks, and that you've been referred for assessment.
Generally, though, I'm not so sure. Thinking back to my pre-diagnosis period - even though I was convinced I would get the diagnosis - I felt wary about saying anything in case it backfired on me in some way. Having said that, though, there weren't many instances where it would have mattered one way or the other. I'm not really sure whether it's right to say 'I have Asperger's' in something like a job application/interview if you haven't actually been diagnosed. I don't think I would have done. But others may think differently.
In your situation with an evening class, though, I would probably have said 'yes' to that question.
Legally you don’t need a diagnosis to be class yourself as disabled under the disabilities act. Hence if you feel there would be a benefit to declaring then yes go ahead