Lolita Shoujo ni Hentai Sei Kyouiku

The protagonist lived alone in his apartment.
Looking out his window onto the street, he had a view of the school.
Every day he saw perky innocent young girls come and go.
They stoked his libido, and as he spied on them from his safe
perch he masturbated to fantasies...
Quietly, privately.
But one day he saw a particular girl.
This story is about what happened next.

[From DLsite English]