Muchi♥Muchi Kyousei Seichouchuu!!!

Based on the CG set/voiced game by Maron☆Maron (まろん☆まろん).

The protagonist is a teacher in a small garden.
One day girls in the garden who ate cookies that induce a rapid physical development become grown-up.
They behave just as their usual days despite the voluptuous bodies...
Receiving lots of their unintentional temptations, his patience is about to run out...

(Source: DLsite English)