Taiiku Souko Monogatari ~Yousei no Natsuyasumi~

Main character has teacher license, but can't find a job. His uncle works as janitor at a private school, and he asks protagonist to substitute for him during summer vacations. School undergoes renovation, and there is hardly any personnel except for the janitor. Protagonist meets two girls who sneak into the school in search of a playground during summer vacations.

[From vndbreview]