Throughout out the series, it's clear that the actors/actresses portraying teenagers are actually in their 20s+. This is done as to not interfere with the schooling of real teenagers and to give the general viewing audience who are teenagers (or young people) themselves someone they can look up to.
A sign in the intro's stock footage reads that Brooklyn is the 4th largest city (by population) in America. Though technically it is a borough of America's largest city (New York City) since the 1890s. However, the borough does have its own distinctions (eg. the people thinking of themselves as different New Yorker type, than those of Manhattan or the Bronx). The sign might also be referring to that the division has more people living in it than all but three American cities (the rest of NYC, Chicago and Los Angeles).
The Sweathogs are supposed to be poor kids, but Barbarino frequently wears some very expensive sweaters.