i'm looking for a manga where a girl just transferred in a new school, because she is shy and then unable to socialize with the new classmates one of them, an another girl, try to help by inviting her to come back to the school at night, then, the two enter a small room where all the males of their class are already naked and ready to have sex with the two girls, the first one is embarassed but the other one is also naked and hugged from beyond; the "veteran" girl explain to the other than this is how she overcame her shyness and became popular with the boys; then the entire class repeat a similar experience at a lake (if i remember correctly) at a school trip