Is there a chance that you feel that you don't deserve to be successful? That you are lacking confidence? Or perhaps you are studying material that is just plain boring. Also some people don't acquire the discipline to maintain good study habits until later in life. Lastly does getting a poor grade cause you to lose focus or does it increase your resolve to improve?

I barely finished high school because of the above problems but when I returned to school (college) a few years later I discovered that much had changed and it all boiled down to my attitudes improving. I still have a problem seeing the long term benefits of anything so I take on-line courses one at a time, which are small enough goals that I easily achieve them.

Having friends who would rather party will definitely have an effect on you unless you can place schoolwork above short term pleasure