Similar to senioritis, summer mode is an epiphany of sorts, typically experienced in the spring season (or closer to June in high school), whereby a student realizes it is nearly summer break, and thus becomes a lot more lazy about schoolwork - skipping class, daydreaming, not doing homework, and not studying for tests or exams.
I can't study for this math exam. I'm already in summer mode.

How does Ms. Smith expect us to write a paper in June? Most of us are already in summer mode!
by Curumno June 16, 2010
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