Zac Efron's mother told him to "have some class" when he flashed fake testicles in Bad Neighbors 2.

The 28-year-old star reprises his role of student Teddy Sanders, who wreaked havoc on Mr and Mrs Radner's (Seth Rogen and Rose Byrne) lives in the first film but is now their ally, helping them battle it out against a hard-partying sorority group next door, led by Shelby (Chloe Grace Moretz).

There were some especially humorous moments on set, such as one scene in which Teddy shows off his private parts. Zac's mother happened to stop by around the same time, making the filming awkward as well as hilarious.

"Those were not real testicles," Seth assured Us Weekly.

"There were a lot of paparazzi on set," Zac recalled. The best was my mom sent me a picture of me, looking right in the lens, holding my fake nuts. She was like, 'Please, what are you doing?'

"I said, 'It's for the film, Mom.' She was like, 'Have some class!'"

This time round, Mac and Kelly Radner are expecting their second child and are a lot wiser having already stood up against Teddy. It was a fun shoot for Seth, who seized the opportunity to relive his youth through the flick.

"I liked going to parties in high school because it was hilarious to see a guy from math class really drunk," Seth grinned of what he misses most about being a teenager. "I'd be like, 'I've never seen you talk and now you're throwing up in the bathtub.'"

Zac has different memories of his younger years, and misses surfing, hiking and cliff jumping in his hometown of San Francisco.

But one thing they could both relate to is feeling "older" than everyone else, with the pair joking they'd talk to each other about "old-people stuff" as they're "dinosaurs". They were even out of touch with the technology the younger people on set would use, with Seth admitting: "The girls would teach us about social media and would show us how to Snapchat."