@Cyzeed
It all depends upon what the school supplied as sex ed, if anything. I don't know what the regulations are in Japan, if they vary from place to place, but here in the US, sex ed in the schools varies from actively detrimental through non-existent and up to fairly useful, depending upon which ideological group controls the school boards and state legislature.
I'll admit, I was damn ignorant about sex when I graduated from high school, as a result of what wasn't provided.