Disclosure Scotland is I understand part of the Scottish Criminal Records Office, so I don't think it's a quango or part of one. It's therefore unlikely to go away. The cost of each disclosure is £23.
Reading my employer's (in higher education) HR website tends to suggest that it's optional. However if I get promoted the disclosure check will be carried out. Promotion just means doing exactly the same job for a bit more money, so quite why I don't need to be checked now but will need to be checked on Monday when I'm promoted to Chief Brain is beyond me.
When I started instructing nearly two years ago I was aware of the Disclosure Scotland thing and raised it with the two clubs I fly from. One said 'not required' and the other shrugged its shoulders.
Either the clubs take the financial hit, or pass it on to the punters but I tend to agree with the sentiments here - the easiest thing is to instruct only adults, but I think that'd be a real shame. I currently instruct a 15 year old, a 17 year old (since he was 15) and a recently-turned 18 year old (since he was 16). The flying really has helped to shape their development as young adults - and I think it's been a unique and beneficial contribution to their character-building.