"Senioritis" may refer to Senioritis, the Broadway-style musical comedy written by high-school students.
Senioritis is a term used colloquially in the United States and Canada to describe the decreased motivation toward studies displayed by students who are nearing the end of their Middle School, high school or college careers. It is typically said to include: slowness, procrastination, apathy regarding school work,a feeling of entitlement or privilege, and a tendency toward truancy. Many high school and college students find themselves in a type of lame duck situation: their plans are made and a new chapter in their life is about to begin, so finishing the current chapter (the current term separating them from graduation) becomes just a formality or "holding pattern."

Background
"Senioritis" also sometimes called "Summeritis" especially when referring to elementary school children is a jocular term rather than a medical one; it is not recognized by the American Psychiatric Association or any other medical body as an actual illness. It is considered similar to affective psychological disorders, such as depression or anxiety. While senioritis is generally viewed as an imaginary disease, its effects are well known to many educators and students alike. Many students feel they are all but guaranteed their place in college, graduate school, or employment. At the high school level, college admission letters arrive in early spring, which affect high school seniors' motivation to push themselves academically for the rest of the school year. It is then said that "Senioritis has 'kicked in.'" A college student may also be faced with job opportunities or graduate school acceptance causing them to lose interest or motivation in finalizing their remaining undergraduate studies.
Senioritis usually results in a withdrawal from school-related extracurricular activities and school-spirit events and a reduced concern on social acceptance, instead focusing on graduation (and the end of compulsory school) in June and/or acceptance to college in September. It can also manifest as increased activity in social and extra-curricular activity, which comes at the expense of academic duties, where the student prefers to "have fun" rather than work on academics.