High school students or college undergrads troop into class and sit down. The teacher or professor will provide a necessary info-dump to move the plot forward, or there will be a bit of characterization business: The main character has forgotten to do her homework because she was too busy fighting vampires, the dreamy kid isn't paying attention and can't answer the teacher's question, the bitchy rival to the heroine humiliates her in front of everyone, the super-smart but borderline-autistic kid invents cold fusion on the blackboard, etc.

As soon as the necessary business is done, the bell rings or the professor dismisses the students, usually with an admonition to read chapters 3 and 4 of the textbook for tomorrow's three minute-long class.

Related to Rule of Conservation of Detail and Moving at the Speed of Plot. Primarily a TV and movie trope for obvious reasons, though it does occasionally happen in prose fiction and comics too.

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Examples:

TV

• Virtually every episode of Buffy.
• Virtually every episode of Veronica Mars.
• Many episodes of Smallville.
• Virtually every episode of classic "school" shows like Room 222, Welcome Back Kotter, Saved By The Bell, Degrassi (and all its spinoffs and sequels).
• Episodes of Big Bang Theory where Sheldon is teaching.

Film

• Green Lantern has a scene like this.
• High School Musical