Choice E weakens his argument because it suggests that the reason public school kids are less likely to go on to college doesn't have anything to do with superior education at private school; instead it has to do with their background and choice--that they want to farm instead . Nowhere does it say...

In that case, answer choice E could be seen as strenghtening the argument as well. Since public schools are right in rural areas, many kids might be thus inclined to quit studying and get in faming occupations.

I honestly beleive this argument is not well compiled !