The text in the background repeats the phrase "Long live the school swimsuit!" over and over.

Is "seduction" is a prerequisite skill for all female ninja?

From Wikipedia's Kunoichi article:

[Kunoichi] training focused on disguise, poisons, and using their gender to an advantage. They would usually disguise themselves as geisha, prostitutes, entertainers, fortunetellers, and the like to get very close to the enemy. It is thought that they would generally seduce the soon-to-be victim and when they get close enough, and would poison them. Kunoichi would also hide weapons in their disguise, like poisoned needles in their hair and knives up their sleeves.