Having worked with some small farms that sell boutique $VEGETABLE at absurd prices I don't think those are the kinds of businesses I want to subsidize with cheap labor (i.e. higher quality than they could otherwise get at that price point as a result of the college credits).

Construction is probably a better choice because there's far more diversity to what the field entails and it provides real world context for many more degree programs than farming does.

Why not offer the kids a choice? Farming offers many lessons beside the agricultural part like economics, managing, husbandry..