Australian superstars 5 Seconds Of Summer have enjoyed a huge career in music so far and their old school's music teacher, Mr. Adam Day, has now opened up about what they were like as his students years ago.

During a new interview with the Sydney Morning Herald, Mr. Day admitted that the lads were always confident in their passion and flair for music, but they have come such a long way: "The last time I saw them perform live for the school was for the Senior Soiree in 2012. They were confident and well-performed, but nothing as out there as what they are doing now.

He went onto add that he checked out the heartthrobs' new live stage show earlier this spring and couldn't believe how successful they have become: "I was stunned when they came on stage in Sydney. They were just different boys with thousands of screaming girls in front of them.''

"I started teaching them music in year 7, and they excelled in all the practical activities of music but were very quiet and shy and reserved - they were very much closet musos. I wrote on their reports back then that it would be good to seek performance opportunities to develop their confidence. They've certainly done that - more than any other student I've ever said that to!"

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Watch footage of Mr. Day surprising the boys here: