As I think, one of the major reason of Senioritis is that Education system creates an Ideal yet Unreal world around students. But when students enter High School, they get to know about real world dynamics and expectations.
This leads to disinterest among students. Also they feel betrayed by people around them for not telling the reality. Hence, they stop trusting these people any more.
Please share if you agree/disagree with this thought. Read more post about Education