SCREEN heart-throb Zac Efron may have turned 21, but school is still very much on his mind. The star of the High School Musical movies goes back to school in his first post- HSM movie, 17 Again.

He is first seen as high school star basketball player Mike O’Donnell, and then fast-forward to 20 years later when Mike – now played by Friends actor Matthew Perry – is in a dead-end job, doesn’t understand his teenage children and is getting divorced from his childhood sweetheart.

Visiting his old school to see his children, he meets a janitor who magically transforms him into his 17- year-old self. Which is where Zac reenters, now playing a 37-year-old, who goes back to school to try to reconnect with his children.

If you say that playing a high school student doesn’t seem much of a change or even a stretch for him, Efron will disagree.

“There were so many roles post High School Musical that were just high school stories. They were regular high school characters.

They were dealing with high school romance and high school scenarios and high school drama,” he says.

“In 17 Again, I had an opportunity to play a 37-year old man, which is someone that I can virtually in no way relate to. And that seemed more interesting.

“I got a chance to play my father, really. That seemed like more fun. And I had a blast doing it, so I’m glad that it came through.”

One of the challenges was doing enough to make people believe he was Perry in a 17-year-old’s body. The two actors worked together to get the same mannerisms.

“Matthew’s face is very specific and his mannerisms are very specific. He’s got, I don’t know, a built-in culture that was fun to try to find with him and Burr (Steers, the director),” explains Efron.

“He was always available on the phone, and I could call him and just ask, ‘What should I do here? I’ve got three things that I might try’. And he would say, ‘Go with the first one’.”

Efron’s father visited the set a couple of times. What did he think of his son’s performance? “I think he was amused.

I don’t know personally what he thinks, He’s just happy. It’s pretty weird for him,” he says.

He admits that if he had the chance to go back to high school, he would probably change a million things.

“If I could go back, I’d just pat myself on the back and be like, ‘Dude, you’re out of here’. It felt like for ever.

High school feels like you’re stuck there for an eternity. And the whole time you have to deal with all these strange social interactions.”

More detail is needed on these “strange social interactions”. He says that means having to sit right behind the prettiest girl in school “and stuff like that”, adding: “You’d be surprised how distracting and how much pressure that puts on you.”

So who was the prettiest girl in his school? “I don’t want to say her name but I still remember her. I’m never going to forget that. There just are things like that – getting called out by one of your teachers in front of the whole class.

“Public speaking in high school is a nightmare. I was actually all right because I was doing musical theatre, but some people would literally get the shakes and not be able to talk in front of the class. It’s very unnerving.”

Sometimes he feels older than he really is, perhaps a legacy of getting along with adults from a very young age. But he also describes himself as very young at heart. “I think I still have fun. I still have fun doing very young things, like watching cartoons in the morning,” he says “I still watch SpongeBob sometimes.

Saturday mornings when I roll out of bed, sure, SpongeBob will probably stay on.” For a young man who has the ability to set teenage girls swooning, you expect that he had a riotous 21st birthday party. Not at all. He reckons it was a pretty low key for such a celebration with only family and close friends. “But it was a lot of people that I’d never seen in the same room before, so I was very excited to have that.

There were probably about 60 people, something like that.”

His 17 Again director Burr Steers was there. He credits him with pushing him in playing the role, and making things interesting. “He would change scenarios and make them real to a degree, and that was fun,” says Efron.

“I learnt a lot from Burr and I feel that 17 Again was a big step in the right direction for me.”

Off screen, he pursues his love of surfing and skateboarding every chance he gets. “It’s harder to go in California. I tend to go when I’m on vacations.”

■ 17 Again (12A), in cinemas tomorrow.