I am going to provide an example:
In a middle class environment, success would be moving up to an upper class living standard and failure would be moving down to a lower class living standard. A person who gives a half ass effort, will meet the requirements necessary to maintain a middle class living. They will get average school grades, take on a solid, but not spectacular, job and live a middle class life. They have done nothing outside of the ordinary expectations of their environment.
Lets move to a Public housing environment. Success would be moving up to something better, perhaps a lower middle class life, and failure would be homelessness or death. A person who gives the expected effort--nothing more, nothing less--will drop out of high school, hang in the hood and take a low income job and stay in the public housing system.
So neither person has been "lazy". Both have done exaclty what is expected of them. The difference is that the middle class person has half assed there way through an environment that has higher expectations.