Many of the comments on the NY Times article I linked to below said that farmers should hire students. My guess is that's more difficult than one might imagine. Any given farmer for any given crop needs harvesting labor only for the period the fruit is ripe. In the wheat belt, I understand that harvesting crews follow the custom harvesters north, from Texas into Canada. That setup is probably better able to attract students than fruit and vegetable. I remember in my youth buses of migrant labor (black or "Negro" as we said then) would arrive to pick beans for canning (just a handful of fields in my area, on the river floodplain). They'd go north for other crops. And the film, "The Ciderhouse Rules", shows a similar setup for apples at a later date. But somehow I can't imagine today's parents of college-bound children encouraging them to follow the crops north as harvesters.
As for dairy, the other type of farming in the piece, not many teenagers are willing to get up at 4 am for first milking.