Senior-itis is an affliction in which seniors no longer feel any motivation to do work in school. They don’t care about what they are learning, they feel it is pointless, they disengage.
At the root, senioritis is a symptom; years of external motivation are the cause. External motivation is a form of motivation that comes from outside - grades, candy, money. Opposite of external motivation is internal motivation - doing something for the joy of learning, to live a morally upright life, or because you value the experiences you will have in the doing.
Students and teachers (ideally) live in a kind symbiotic relationships. Students trying harder makes teachers want to teach more. The better a class goes, the more time is spent on preparing for it - by students and teachers alike. Seniors who give up on learning break this virtuous cycle, leading to sophomores not trying to understand what is going on in a class - giving up years before they are really done with school. Quitting that early means they will be unprepared for any future classes, since they don’t really have half of the work from their sophomore year.
(Possibly unrelated to the above.)
After gaining a small amount of technical skill, sometimes people are tempted to quit improving. When Picasa was a teenager (14 years old), he could already draw very well. Imagine if he had quit, satisfied that his work was better than other kids his age.
Computer science is an art as much as technical skill. You should strive to write short, elegant code. Code full of ugly hacks, “just get it done” work cannot be used to build something complex. The burden of all of the code, the inappropriate levels of generality all mixed together, makes it harder and harder to think about the goal you intend to reach.
In practice computer programming requires a high level of abstraction and organizational skills. Work to develop them. Designing and writing a (kind of simple) game is a good test of your skills. Try it!