Compulsory National Service right out of high school seems to me to be a terrible idea, for several reasons.
First of all, many of these kids are planning to go onto college, and it is not a good idea for them to have such a gap between their secondary and higher education. Many students who take time off from their educations never go back, or have a tough time getting back into the swing of things when they do. Most people I've known who have taken years off before completing university have had a lot of trouble getting their degrees (and some never did). There are a lot of skills and knowledge that kids carry over from their high school years, and they run the risk of forgetting these things if they take time off.
Second, forcing people to work for under-market wages goes against the entire idea of a free market economy. Conscripted labor will artificially depress wages in whatever jobs these kids take.
Third, the idea that kids need to participate in service to make them better people is one that hasn't yet been proven. How do you know that 2-3 years of forced labor will make young adults more mature and better rounded?
Fourth, there are programs that already give kids these opportunities, such as the Peace Corps, Americorps, and Teach for America. These programs turn away volunteers every year. Why not expand these programs and see the impact on the country before forcing everyone to participate? Maybe just as you don't want to see unqualified people in the military, service programs don't want them either. Do you really want someone who couldn't pass the ASVAB to build your house??
Last, the idea that teens should be forced to "volunteer" for their country completely ignores the fact that many teens are already volunteering time to plant trees, build houses, tutor children, and do many other things for the community. Kids today volunteer more hours than any generation before them.
Let's support today's kids and teach them good citizenship, instead of taking away their personal freedoms. This country was founded on principles of personal freedom, and we should not undermine that message by conscripting young people into any type of forced labor.