My 16 year old son has ASD which was diagnosed a year ago. He has had problems in school since about year 3 in primary school. We didn't see it then, just thought things would get better when he went to secondary school. They didn't get better, they got worse. He became more and more isolated and reluctant to go to school. When he was in year 8, I asked them to assess him for ASD - they laughed at me and told me I was making a fuss about nothing.
So we moved him in year 9. The new school were a lot more receptive, but the problems continued (even though he liked the new school a lot more) while we waited for assessment. He started having panic attacks and refusing school. Eventually, he was diagnosed last summer between year 10 and 11. He managed to do well in his GCSEs despite about 70% attendance, largely due to the support from the school and the excellent teachers.
He's now moved to a small independent school for sixth form because he just couldn't cope with being in a large comprehensive school any longer (and thankfully is a lot happier).
The last three or four years have been hugely stressful for us as a family. Dealing with his anxiety and school refusal, never knowing if we were doing the right thing for him or not.
I strongly feel that if someone had spotted it earlier, or if the original school had listened to me when I first suggested it, a lot of the problems would never have happened because he would have got support earlier.
Kids with ASD often function fine in primary school, then run into problems when they start secondary. Just because a child is OK now, doesn't mean they will continue to be fine.
Because of my experience, I would always recommend getting a diagnosis. It could avoid so many issues in the future.