Wise words.
I have found it to generate the best productivity and general satisfaction among my employees to set goals with them that are within their reach, stretches them individually, contributes to the overall success of the organization, and then identify and reinforce the right behaviors to help them reach those goals. If they have a part in the goal setting and buy into what we are trying to do, then it builds more personal satisfaction in each of them. This also allows for their individual differences and lets them grow in ways meaningful to them that still has an impact on the bottom line. Of course it cannot always be that way, but if that is the general framework in which we operate then telling them to "work hard" gives them not only a frame of reference, but also a clear understanding of what hard work and good results look like, as well as the rewards that are thereby attainable.
I think this same general idea would work well for students too. We have used this method with our kids generally and they seem to have gotten more out of school, even when they weren't the top kid in the class. Even with our 17 year old who is dealing with epilepsy and struggles mightily to even perform at basic passing levels. At least he knows what hard work can get him, what it means to work hard, and how to translate that into his personal circumstances. I think that is half the battle.