They were lazy, found school boring, or they just never applied their skills. Some of these people did poorly in the education system, but became very successful later in life. In other words, they worked hard and were able to get ahead. Academic skills can be, but are not always, a predictor of recovery. A lot of studies have found that use of drugs and alcohol is a negative predictor.
Alcohol kills more than any other drug.
Of the 50,000 traffic deaths per year in America, half are alcohol-related It is clearly important to stop using alcohol and drugs following the injury. Alcohol especially tends to attack areas of the brain dealing with short-term memory. Those who continue to use drugs following a head injury have a poor outcome. Each doctor you talk to is likely to give a different estimate of how long it takes to recover. Some physicians may say that recovery from head injury is 6 to 9 months.