Senioritis, it's even on Wikipedia

Senioritis, from the word senior plus the suffix -itis (which refers to inflammation but in colloquial speech is assumed to mean an illness), is a colloquial term used in the United States to describe the decreased motivation toward studies displayed by students who are nearing the end of their high school, college and graduate school careers.^[1]

Causes/Effects, Symptoms

The main symptoms of senioritis include chronic procrastination, lack of motivation, a drop in academic performance, and "coasting," which is the act of going through classes with very little concentration or application of intent. This usually happens in the last year of high school, college or graduate school. One of the most notable symptoms of senioritis, is that all attempts by educators to curb senioritis tend to actually increase senioritic symptoms. High school seniors experiencing senioritis after admittance to college are fed up with high school; as they see it, they have achieved high school's goal of getting into college and don't understand why they are still expected to work. Not to mention, most seniors, even those accepted to top-ranked institutions, do not love academics as much as their college essays would seem to indicate. Therefore, when a teacher attempts to motivate afflicted seniors, their efforts are only met with resentment.

Senioritis has been known to surface at different times depending on the personality of the afflicted individual. The most common time for a senior in High School to develop senioritis is at the beginning of the second semester, after all applications to college have been submitted. In some cases, senioritis can surface at the beginning of senior year. These student have a particularly hard time focusing, as their senioritis is usually triggered by being fed up with the administrative shortcomings of their High School. Senioritis typically begins during the senior year, (occasionally signs of infection begin to show themselves during fourth quarter of the junior year) there have been some extremely rare cases where individuals are born with senioritis. Individuals born with senioritis tend to make excuses as to why it is "ok that they don't care" because "it doesn't matter all that much anyway." Such individuals are extremley unproductive for all of their academic years, and ultimately their lives. They also tend to be extremely infectious to other seniors who become prone to senioritis during senior year.