Roughly speaking, 2 years of community service should be worth some % of local & federal taxes.
In a developed society, I'm not sure what kind of labor an 18-year old can perform (what I mean is that it would be mostly unskilled labor), that would be better than taxing this same individual later in life, without delaying their education by 2 years.
I suppose there would have to be exemptions for college students as there typically are in such schemes in other countries?
Shouldn't be. Just have a more-level playing field, make everyone (college-track or not) participate.
You seem to assume that tax dollars are equivalent to labor. A pile of quarters never did anything sitting there, it takes a human to do something for the most part. Money is a tool sitting there, not actual work.