I'm pretty sure I had one of the worst cases of senioritus ever and it started middle of my junior year! Much like footballmom said, I started working sophomore year and realized pretty quickly by watching my senior friends that running on the hamster wheel to get all A's in the honors math & science track wasn't worth the effort, since they were all getting into the schools I'd apply to with less than stellar effort. My dad got ill during my junior year and that changed what was important as well.
During my senior year I had 39 absences (we were allowed 10 per marking period; it was changed shortly after I left
and I walked into my GC's office on my 18th birthday to ask if the colleges I was accepted at would take a GED in lieu of a HS diploma. Poor new GC; guess it wasn't a question she was expecting one of the NMSF's to ask. I finally agreed to finish out the year because it wasn't worth the hassle of dealing with the GED fallout. But I was done with all the HS bs and wanted to get on with my real life.
I turned out fine. Received my BA, earned a Masters with a 4.0. And I've found myself telling my D to "get off the hamster wheel" quite a few times during her junior year.