DS1's diagnosis (age 7, medicated at 8) made a huge difference to him once he got to the age where he understood what Adhd was. And then he was medicated and the difference has been marked... he is now 13.
His school work suffers when he doesn't take his medication as he simply can't focus long enough, but he's an intelligent young man. He also finds it easier to use supportive strategies with medication as it gives his brain a chance to slow down, stop and think before acting on the impulse. Ds is also suspected to be autistic, so his adhd conflicts with his need to follow the rules when it is not being well managed.
On the whole, for Ds diagnosis has only been a positive. He's not allowed to use it as an excuse for poor behaviour, but when his teachers start complaining that he doesn't focus in lessons at the end of the day (when his medication has worn off), that he sometimes answers back and is more challenging to redirect, it's a reminder that Ds does see and respond to things differently to many of his peers.
Primary school was a bigger struggle due to constant changes to the structure and routine. The regularity of secondary school has been hugely beneficial to him.
Dd2 on the other hand... had an unexpected diagnosis of adhd and unfortunately a trial of medication showed it was not particularly effective for her. She was 7 when she got her adhd diagnosis. She also has social communication disorder on top but also believed to be autistic.
She starts secondary this year and she is struggling massively. She does not know why she feels and reacts the way she does. She doesn't understand why she is different from everyone else and just wants to fit in. She does not.
Academically, she is very capable in But she struggles to understand new concepts and needs 1:1/small group work to embed them as once she doesn't understand, she gives up.
In terms of getting a diagnosis and support, your school SENDCO will be able to gather information to make a referral for assessment. If you have no joy with them, the GP can initiate the process but school will have to be involved - Ds' diagnosis process was raised through GP from a very early age. It took 6 years from first raising concerns about development to get to that diagnosis. The school we were at during that time were not helpful or particularly supportive until their hand was forced. The second school were brilliant from day 1 and were the drivers for dd2's referral and diagnosis. They continued to push for autism assessment right up until the end of year 6 believing strongly that she is also autistic.