Private NPC Sex Academy (Update Android ver)

According to school legend,
any confessions under the big tree on the school grounds will lead to eternal happiness.
The protagonist made his confession, and started happily going out with the girl he liked.
That’s how it should have continued.
Suddenly, his girlfriend becomes capable of only repeating the same lines over and over.
And not just her, but soon everyone in the school is like that.
Can the protagonist grasp happiness with this new set of circumstances?