Gokko Vol. 02 School Gal's

Gokko Vol. 02 School Gal's is very similar in structure to the previous volume, but - as the title implies - with a different theme. The player controls a young male protagonist (default name Lance), who has spotted a beautiful girl near the gate of the prestigious Finlandia Girls High School. He is determined to gain access to the school, which is also attended by his sister. Though men are strictly prohibited from entering the school building, the hero is resourceful enough to find a way in; what happens afterwards should be clear to anyone familiar with the many high school-themed bishoujo games...

[From Moby Games]