In several episodes of Assassination Classroom (animated version), characters develop spontaneous nosebleeds seemingly in response to becoming sexually aroused. This occurs with a number of male characters, though I can recall at least one instance where it happens to a female character as well.
Here's an example on Youtube:
Is this a common trope in anime, or something that's unique to Assassination Classroom? And in either case, is there any rational explanation behind using this seemingly bizarre method of depicting arousal?