You can't 'self-diagnose' for the purpose of school and SEN support though, there is a lengthy pathway ...
Are you talking about SEN diagnosis in school (which the OP refers to) and support given around behaviour difficulties or is this 'self-diagnosing' something you have encountered in another context?
I've said I agree ADHD exists. I know there is a proper pathway for diagnosis.
I perhaps didn't make it clear, but I think some of the reasons why some people in society doubt ADHD exists or think it's exaggerated is because some people are quick to self-diagnose online (along with a handful of other conditions), or they're very selective with when they claim it. As a result, it can lead to people doubting whether the condition exists.
For example, my school has always had great support for children with ADHD. We've also got additional staff employed in our SEN support base trying to push through assessments, though the system can be complicated and that's a different story. When fidget spinners were a trend, dozens of students suddenly presented with notes from home saying their DC should be allowed their fidget spinners, it helps them concentrate, their DC probably has ADHD etc.The parent never mentioned ADHD before, and when school held the rules and the trend died, the parents never mentioned ADHD again.
Every time people do things like that it's an insult to those children who do need additional support for ADHD to have people grabbing the label when it suits.
The ones who say "it didn't exist in my day" wind me up the most. It did exist, but people just suffered and got labelled as bad instead.
This winds me up.
I bet they never say 'nobody had X cancer in my day... Nobody had COPD in my day...' although one of my relatives thinks I can't be asthmatic because I'm healthy and describes my asthma attacks as 'getting a bit out of breath'.