Whats the name of this movie

hi i have been trying to remember what a name of a horror movie is

it was around 2010 when i watched it in mexico

i can remember i think some kids came out of a party and this old man with a bat of wood came up to them with it and hit they got knocked out and took to a asylum of sorts after a while they turned into zombies

i can also remember they went to a school i think the survivors to turn on the schools water that killed the zombies

Hey guys, I didn't see anything like this mentioned before, I've been trying to figure out what this film is for years, I would like to watch it again just for curiosities sake.

I remember seeing this film in the late 90's, though maybe it was actually released in the 80s. It's definitely a horror movie. All I remember is this climatic scene where a teenage girl is fighting off an older woman (stepmother?) by the pool, and the woman has her pinned down and is trying to stick her tongue down her throat, but the tongue is deformed. I don't know if the woman was a monster or alien, I just remembered the scene took place at night and I don't know if she was trying to possess the girl or eat her or what, but I remember seeing it as a kid and am curious what that film was about because the scene was so weird. I believe the older woman is killed in the same scene (by the boyfriend?). I want to say that the woman was some sort of evil witch or something, but my memory is so fuzzy and for some reason that scene sticks out to me. I think the older woman is wearing some sort of night gown or robe. Maybe she had red curly hair?

I love old horror films from the 80s and 90s, I thought maybe this was Stepmonster, but I looked up the synopsis for Stepmonster, and that doesn't sound like it at all. Anyone seen this film before?