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
I would hope, if the evening class is with a recognised educational institution or an organisation like the WEA, that you would be treated with respect - not worse, nor as 'weird', but as any other disabled person would.
I feel happier having my diagnosis for precisely that reason. If anything, I think I was treated worse and as weird before I had the diagnosis - because I had no other way to explain why I did certain things a certain way, and why my behaviour was what it was.
Of course, there will always be people with whom it won't compute. Just as there will always, unfortunately, be homophobes, racists, sexists, fanatics, etc...
As soon as I realised I was autistic, I told everyone, which was 18 months before I got the diagnosis. I also applied for benefits etc and confirmed that I was autistic. And if anyone treats you badly because of it, they’re discriminating you which is against the law. I think it’s helpful to disclose it because if you do need any support or need to leave the room or whatever due to sensory overload, they will understand and you won’t have to give a big explanation as to why. As Hani-andis said, you don’t need a formal diagnosis to identify yourself as autistic, and I also like to tell people because why not. If we don’t tell them who we are they won’t know we exist and I somehow don’t have the ability to not tell them. Lol!