Assassination Classroom (2013 anime)

A 2013 OVA based on the Shonen Jump manga Assassination Classroom.

Koro-sensei is a dangerous alien that has promised to destroy the world in one year. The governments of the world frustratingly tried to kill him but their methods failed again and again. Finally, they put the alien to teach high school students. No, this isn't a plan to make him feel pity for mankind. His students are tasked with the mission of killing him.

This OVA, however, is just a Breather Episode, involving the class going in a school trip with their teacher, having little to do with the overall main plot, that would be pretty hard to fit in such a short running time(31 minutes).

Directed by Keiji Gotou.

Tropes used in Assassination Classroom (2013 anime) include:

Barehanded Blade Block: Despite being made of a material who he absolutely can't grab directly, Koro-sensei is perfectly able to stop a bullet shoot by Red Eye using two pieces of yatsuhashi to protect his fingers.
Cheshire Cat Grin: Koro-sensei default expression is this; he is also a alien that will destroy the world when the times come no matter what.
Crazy Prepared: Koro-sensei travel guide contain instructions about what to do in extremely specific situations, including feeling lonely by seeing couples bonding. It's also humongous.
Death by Sex: In a flashback, Bitch-sensei is shown killing a target while she's on top of him. In a bed.
Death Glare: Karasuma forces Bitch-sensei to change her clothes with one of these.
Friendly Enemy: Koro-sensei and his students respect and even care about each other, but the students will try to kill him or help snipers in shooting him.
Hot Teacher: Bitch-sensei, though she isn't actually a teacher by formation, but a hitman assigned to the class to help them in killing Koro-sensei.
Kryptonite Factor: The material who can hurt Koro-sensi is perfectly available; the problem is fatally hitting him with it.
Neck Lift: Koro-sensei does this to a bunch of delinquents he beat up.
Not So Different: Ryuuki offers Yukiko the chance to join his group because she's a rebel like them.
Paint the Town Red: Red Eye says he will do this to Kyoto in his intro.
Paper Thin Disguise: Koro-sensei disguises can be actually resumed as him using accessories.
Proper Tights with a Skirt: Yukiko Kanzaki wears the school uniform along with black tights.
Sensei-chan: Bitch-sensei is very immature and the students(and Karasuma) don't treat her with respect, with the latter even forcing her to change her clothes for the school trip by acting like a stern father.
Super Speed: Koro-sensei is said to move at a speed of Mach 20.
Suspiciously Specific Denial: Koro-sensei says he's not interested in school trips while a giant pile of baggage is by his side.
Unusually Uninteresting Sight: No normal people seem to care about the obvious yellow tentacle monster walking around, despite the fact Koro-sensei is doing (minimal) work to try to hide.
We Can Rule Together: The delinquents who kidnap Yukiko and Kaede offer Yukiko to join them.
Would Hit a Girl: The delinquents have absolutely no qualms about hurting girls.