This was one of the city's early 20th-century farm gardens, where, as a vehicle for personal development, children learned how to grow crops and flowers. Fannie Parsons, the pioneering force behind the program, put it this way: "We teach them honesty in their work, neatness and order, justice as well as kindness to their neighbors. I assure you that all the virtues can be taught from a little patch of ground not eight feet square."
After enjoying decades of popularity, the movement finally lost its steam, and by the 1970s all the farm gardens in the city had closed. This one in Highland Park, however, was revived in 1989 for the students of PS 140 here in Brooklyn, and it remains open to this day, although it seems to be more of a family garden at this point.
In reading about the farm garden program, I came across a couple of great historical images: this 1936 photo of kids hard at work here at Highland Park, and this terrifying Children of the Corn shot from a Manhattan garden in 1902.