The Ohio high school’s class trip to Washington, D.C. was a bit of a bust. Until, that is, one classmate had the chutzpah to get on a pay phone, make a call and make a pitch.

Before they know it, he and a handful of his classmates are shaking hands and sharing a a few life-goals with the vice president of the United States.

Cute story, something to tell your own kids decades after it happened, right? That day in 1978 when you met Vice President Walter Mondale. And the class joker who made it happen? That would be the weird kid in school, the solitary blond named Jeffrey Dahmer.

“My Friend Dahmer” is a “Dazed and Confused” look back at high school in the ’70s — the hideous fashions, the ugly decor, the high school hijinx that a group of band nerds/tennis team pals got into.

But hanging over it all is the darkest of shadows. That awkward kid who never quite fit in, who got attention by faking spastic fits for laughs in class, in the halls and at the mall? He become one of the most notorious and gruesome serial killers in American history. It’s “Dazed and Confused” with a chilling true-crime edge.

Every mass murderer makes us reach for answers. The spree killer who shoots up a church, a concert, a nightclub or an elementary school, the methodical, one-murder-at-a-time serial killer who develops a taste for taking lives, the genocidal freak who oversees the slaughter of thousands or millions — there has to be a “reason,” right?

“My Friend Dahmer” gives us one of the most fascinating portraits of a serial killer, ever. We meet young Jeffrey (Ross Lynch of Disney Channel’s “Austin & Ally) his junior year. He sits alone on the bus. The only kid who speaks to him in school is also the only kid more bullied than him.

Hey, you didn’t let the fact you were a Neil Sedaka fan get around in the ’70s.

His father (Dallas Roberts) frets over the kid’s lack of friends, the time Jeffrey spends in the shed out back, his “lab.” Because Jeffrey’s become engrossed with picking up road kill and dissolving the flesh off it with chemicals dad brings home from work.

When we meet Jeff’s mom (Anne Heche), we understand the father’s worry. She’s flighty, flaky and impulsive. And she’s just a month out of a mental hospital.

But maybe environment can trump genetics. If Jeff would just join different clubs, “get out of your shell,” make friends, perhaps this story could have a happy ending.

“My Friend Dahmer,” based on a memoir by a guy who knew the killer as a kid, reconstructs that last year, when Dahmer became a prankster — inviting kids to laugh at his “fits” (fake), forming friendships when they pick up on his ingenious performance-art style stunts.

The new pals (Alex Wolff, Tommy Nelson, Harrison Holzer) work on the yearbook. “What if I…was in every (club) photo?” The image of the creepy, blank-faced Dahmer staring out of Honor Society, Spanish Club, Debate, etc. shots for all eternity is too hilarious to pass up.

Marc Meyers’ film zeroes in on speculative material about Dahmer’s home life — an unstable mother, a father overwhelmed and ready for divorce, both parents’ preference for Jeffrey’s more “normal” younger brother.

His sexual awakening is a confused blend of peer-pressure girlie magazine ogling, seeking acknowledgement from female classmates, contempt for mother’s effeminate (and spastic) decorator, and a growing obsession with his fit, jogging doctor (Vincent Kartheiser).

Nothing here suggests someone who has “just snapped.” We witness a deepening, disheartening antipathy for life, human or otherwise. God forbid this kid should come into contact with pets.

Lynch is deadpan perfect in the title role, an academically under-whelming, socially-awkward and musically mediocre trumpet player battered from all sides, with no help in sight. High school eventually gives him a quantum of solace, but whatever inroads he makes socially (even bullies appreciate a good fake spastic fit), the pall that hangs over the picture ensures that we know it’ll never be enough.

Heche doesn’t let us hate the mother. Madness isn’t something you can blame somebody for. Roberts’ father-figure has the right worries even if he prioritizes his own happiness. And Dahmer’s high school experience isn’t a “Carrie” horror that aptly explains what he became.

Which, in the end, hamstrings this thriller. We can only know so much, and the guesses don’t quite add up to anything “textbook.”

But “My Friend Dahmer” tallies a bleak scoresheet in the “nature (crazy mom, crazy kid) vs. nurture” (bullied kid, absent father, morbid hobbies) debate.

Whatever path Young Jeffrey was headed down, a lack of competent, unquestioning love — from parents, from any healthy romantic interest — made certain he never strayed from it.

MPAA Rating: 

Cast:Ross Lynch, Anne Heche, Alex Wolff, Dallas Roberts, Vincent Kartheiser

Credits: Written and directed by Marc Meyers, based on a Derk Backderf memoir. A FilmRise release.

Running time: 1:47