Yobs start at a very young age, this will not get rid of yobs.
Government will still hand out money, so yobs will do their time have a bigger chip on their shoulder and go get the handouts.
So you are essentially saying once a yob is a yob then they can't be "saved"? Well nice on you...
Of course governments will still hand out money, the point being they will most probably hand out less as at least some of these yobs would actually have some skills and be useful for society again.
It will effect people, Plenty of people study till 27-30 years old. School holidays have to be used to get money.
Fix the problem, don't introduced forced service for everyone that will not fix the problem and just annoys everyone.
I know, my GF is one of them (along with quite a few of my friends), however the national service I advocate wouldn't affect that study. The national service would be during the summer, when they aren't studying, and if they are (I spent a lot of my time off on fieldtrips) they can be included/national service can work around that.
As for summer holidays needing to be used to make money, true for a small amount of people that really need the money to keep studying, the rest either use that money to splash out on holidays or to live more comfortably (this is from personal experience with friends, I have been through the whole uni thing and know how it really works). Having said that who says national service doesn't pay? I assume other nations pay their national servicers at least some form of pay and in either case if money was desperate (funnily enough the only person I knew that really needed money to stay on at uni was essentially middle class) they could spend their time working with a company at least part time, as mentioned previously.
It does not solve the problem
you are implementing a national structure that effects everyone yet your problem area is what 1% of the population or less.
solve the PROBLEM don't implement a rubbish system that will achieve nothing.
I'm not saying don't fix the problem to begin with, and not saying national service should be just for yobs. I'm saying national service would be good for most people going through it, giving them experiences they couldn't get otherwise as well as experience in life and jobs and useful qualifications/skills. I think it would help a lot to remove some of the biggest issues employers have with young people, especially those coming out of uni.