ADHD updated NICE guidance 13.3.2018 states that:

 The effect of environmental factors must be fully accounted for and appropriately adapted prior to a diagnosis of ADHD being made.
2. A behavioural baseline must be established.
3. Parents of under-5s must have completed a recognised and accredited parent training course before a diagnosis of ADHD is considered.
4. The advice of another qualified tertiary professional e.g. an Educational Psychologist, should be considered before a diagnosis of ADHD is made:

NICE Guidance 2018

See also:

Dramatic rise in ADHD diagnosis

And what Educational Psychologists think about this:

Educational Psychology perspective.