Didn't even need the ghost
Good movie, but the ghost angle was superfluous. It didn't add much to the main story, especially since Jaime is the one to provide the flashback of Santi's death. And the kids overpowered Jacinto on their own. The ghost just provided a karmic bookend.
The supernatural elements aren't a bad idea per se, but I think they were too blatant about it, what with Santi's ghost just showing up on screen and bleeding all over the place with his gaping head wound. I think it would've been better if the movie never showed him so concretely so as to keep the audience guessing if he really exists or if Carlos is just spooked from the other kids' ghost stories. Maybe only show his arm reaching up from the depths to pull Jacinto down at the end. Just my two cents though.