Spiritual Bind ~Rakuen no Gen'ei~

Main character investigates old school building with his friend to find the source of "curse mail". He is joined by some other students who want to find lost dog. Together they encounter paranormal activity that they need to oppose somehow, and strangers help them to overcome the danger. However, on the way back from building they get engulfed in darkness and appear in another world inhabited by hostile demons.

[From vndbreview]