I think that they should do one year of service, but they should be able to choose what they do, so that it doesn't seem like a chore and more people do volunteer work and love it. Part of the International Baccalaureate Diploma Program is that we have to do 50 hours of Service throughout the two years, but we can choose what we do, which is the organisation's mandate to create well-rounded world citizens. I think that could be very easily be integrated into most curricula.
In most high schools here, they have to perform a certain amount of hours of community service in order to graduate. The number of hours has varied from 20 to 50 depending on the high school.