In an abandoned high school in downtown Atlanta, Charlie Day and Ice Cube are prepared to duke it out in a classroom.

It’s an unfair fight of course, pitching a quivering Day against the rock solid Cube in a David vs. Goliath scenario between teachers, and the premise of new comedy Fist Fight. Directed by It’s Always Sunny in Philadelphia’s helmsman Richie Keen, Day is counting on his buddy to make sure he doesn’t end up in an ambulance.

No stranger to adult comedy, having switched from small to big screen courtesy of Horrible Bosses and Vacation, Day feels familiar enough with the genre to serve as an executive producer on Fist Fight, even coaxing his pal Christina Hendricks to play a sexy French teacher.

Day already looks a little punch drunk when he chats with STACK on the set – and that’s before the fight has even begun. “I’ve essentially gotten Ice Cube fired,” he explains. “He’s had an incident with a student, and he’s asked me to stick together with him, saying that ‘Teachers always stick together’. But because my job’s also on the line, I rat him out, and he challenges me to a fight at the end of the school day. I spend most of the movie trying to worm out of it, and then I man up and fight him. And it doesn’t go too well… but I give him a better run than he expects,” he says, attempting to flex a non-existent bicep.

Recalling his own high school years, he says, “Oh, it’s terrifying, isn’t it? Especially in a school like this, which feels like a prison. High school was not the happiest time of my life, so it brings back chilling memories, so I had all those feelings when we begun filming, but now I’m numb to it.

“I struggled in school. I couldn’t pay attention. I blame the girls. I was too distracted. Better as I got older, but as a youngster, my head was in the clouds,” says 5’ 6” Day, 41, who was raised in Rhode Island, the son of two music teachers.

Fist-Fight

He learned fast. “I got into a few scraps, but when you’re as small as I am, you try to avoid them, and I think that’s how I learned to be funny, which was ‘I got to make these guys laugh, or they’re going to knock my teeth out!’ ”

Fist Fight, he says, is set in the high school of nightmares for any sensitive kid. “It’s not exactly a quality school,“ he offers. “The school is struggling and the teachers are at ground zero, stuck between an oppressive administration and out of control students. Ice Cube’s Mr. Strickland has a militant philosophy of how to deal with these kids, whereas my Mr. Campbell tries to kill them with kindness, and those two philosophies have to meet somewhere in the middle until the final showdown.”

Featuring Tracy Morgan, Dennis Haysbert and Jillian Bell, Christina Hendricks’ Miss Monet is the teacher who every kid crushes on.

“Miss Monet certainly has a dark side. She carries a switchblade in her pocket and is ready to use it,” purrs the Mad Men star, who has gone on to find her funny bone in Bad Santa 2 and Zoolander 2. “They wanted me to go with the idea that she fancies herself as this worldly woman who pretends that she’s a bit French but is actually just a girl from the Mid-West teaching French. I don’t even think Miss Monet is her real name,” she laughs.

Like Day, Hendricks, 41, has few happy high school memories, shuddering as she walked on to the real high school set on her first day. “I hated it. I hate high school vibes. It gives me the heebie-jeebies. When I first walked in, I was like ‘Ooh! Someone’s going to make fun of me!’ ‘Someone’s going to say something mean, I just know it.’ It’s a psychosomatic thing that happened immediately. These guys are great but it takes me right back to high school, just waiting for the bell to ring so I can leave.”

Fist Fight is in cinemas on February 23.