Teenage brains. Alcohol. Driver’s licenses. Put them together, and what you get, inevitably, are a whole bunch of bad decisions — in real life and in movies.

It’s not a new lesson, but it’s the most coherent one in “Ma,” a middling high school horror revenge fantasy (if that’s a thing) starring Octavia Spencer. Oh, here’s the other: that Spencer humanizes and improves pretty much every movie she’s in.

The Oscar-winning actress is the only reason to see “Ma,” and she’s clearly having a grand old campy time playing Sue Ann, an unstable middle-aged woman who clearly hasn’t, um, graduated from decades of resentment toward the cool-kid bullies who mistreated her in high school. To be fair, she has a right to be angry. Like, really angry.

Spencer is directed here by longtime friend Tate Taylor, who also helmed “The Help,” for which she won her Oscar. When Taylor presented her with the idea of a horror film, he has said, Spencer noted that black characters tend to die in the first 15 minutes of most horror films. To which Taylor replied that not only would Sue Ann not die at the beginning, but she’d get to kill a lot of people, too. And so she does.

But we’re getting ahead of ourselves. We begin as young Maggie (Diana Silvers) and her now-single mom, Diana (Juliette Lewis), are moving back to Mom’s nondescript old hometown, U-Haul in tow.

Maggie’s the new kid in high school — never fun — and starting on a Friday, no less. It helps, though, that she looks like a supermodel and a young Nastassja Kinski (didn’t we all, in high school?)

We try to ignore the rather obvious “Dead End” sign near their modest new home. Diana gets a waitress job at the local casino, and Maggie, after maybe four minutes of loneliness, makes friends with the cool kids. Doing so requires suspending her better judgment and getting in a van to go drink by the dirty rock pile outside town. (Fun times!)

But first, they need the alcohol. That’s where Sue Ann comes in.

The lady walking by the liquor store SEEMS cool. A vet’s assistant by day, working for Dr. Brooks (Allison Janney, predictably hilarious, even when just asking Sue Ann to answer the phone), she seems to the kids to be a godsend. She apparently lives alone and has a great basement where she invites them to drink and party as they please. She has a few rules, though. The most important is they can NEVER (never, ever) go upstairs. That’s Sue Ann’s private space.

And so, nobody goes upstairs, everybody stays safe and they all live happily ever after.

KIDDING!!

These are teenagers, remember? They keep on partying even after Ma, kinda sorta joking, pulls a gun on one boy and forces him to strip. They keep coming back even when Ma reveals herself to be desperately needy, hungry for attention and clearly unbalanced.

Because, hey, she has a basement! And booze! And she dances the Robot! “Now you know where the party is,” she says. Yep, they nod.

Only Maggie, it seems, has any sense of foreboding. But even she can’t imagine what lies in store once Sue Ann has figured out who the parents of some of these kids are — the same kids who treated her so badly in high school, of course.

Let’s just add, the teenagers here have no monopoly on bad choices. “You’re a loser — you always have been,” remarks one of the parents to Sue Ann. Now, is that a good thing to say to a woman you know is hanging with your kids AND has unlimited access to animal medication?

Taylor himself appears in a cameo as the police chief who confronts Sue Ann, with less-than-ideal results. It all comes together in one gory finale, where Sue Ann makes perfectly clear that she’s prepared to go as far as she needs to because, as she says very believably: “The humiliation never goes away.”

Revenge, the saying goes, is a dish best served cold. To which Sue Ann might add: You’re never too old to serve that cold dish. It would be nice if high school bullies of today would take note.

But they’re teenagers.