Wonderful idea! And salmon live in trees and eat pencils for their lunch.
Bear in mind that this government (and many of its predecessors of both persuasions) has been unable to get large numbers of people to do any useful work at all even for payment. Furthermore, many young people cannot be persuaded to attend school until they reach 16 (soon to be 18) as the law requires.
There is ample opportunity for young people who want to undertake work in the community to do so. Many of them do, most of them don�t.
The idea that all under nineteen year olds will suddenly take to the idea of working for nothing doing things in which they have no interest is somewhat optimistic. The notion that benefit payments will be stopped for those that fail to comply is equally far fetched.
The cost of administering (and enforcing) all this? Unknown, but likely to be squillions of pounds. The benefits of forcing people to do things they don�t want to do (and presumably penalising them if they don�t)? Also unknown (but likely to be closely adjacent to nil).
Sure sign an election is due next year.