It sounds like an expensive, administrative nightmare that would never be implemented effectively. It would cost the country a lot of money for nothing. Exactly which organisations want to support 18 year olds in the workplace, for one weekend per month, who don't want to be there?
Do the armed forces really want to spend their budget on training a bunch of teenagers who are going to leave before they are useful in any way? What nonsense.
I also don't believe that anyone should be forced to work for free. That's slavery. It's not 'voluntary' if it's forced.