Prince Harry and Meghan made an unannounced visit to Macarthur Girls' High in Parramatta on Friday, with only the head teacher, her deputy and the groundsman were supposed to know.

They arrived at Macarthur Girls school in Parramatta shortly after HSC students at had finished an exam on Friday.

NSW Premier Gladys Berejiklian was the warm up act.

"Most of you would probably have got up this morning, got ready for school and turned up thinking it was going to be a normal day at Macarthur Girls School. Is that right?" she told a special outdoor assembly.

The answer was unanimously "No."

The premier added: "You knew I was coming. Did you know anyone else was coming?"

"Yes".

But the whole school erupted in shrieks and cheers as Harry and Meghan walked into the assembly.

They first watched a group of dancers perform to the Frankie Goes To Hollywood version of The Power of Love.

Meghan looked fantastic in Roksanda navy dress with light blue trim and high heeled beige shoes.

The Duke and Duchess of Sussex then spent time talking to to the year nines about the girls' social justice project and youth empowerment.

They also heard about the NRL's League in Harmony program which aims to unite and empower young people to be advocates of positive change in their communities.

Maithly, 15, said: "It was a lovely experience and beyond anything I would have ever expected."

Rhiannon, 15, said: "When they walked in I felt like my heart stopped. Their presence just made everyone shocked. I never thought this would actually happen at our school."

It was all over in just 30 minutes but the girls were blown away.

Frank Wilson, groundsman, said: "I put in an awful lot of work for just a short visit."

Australian Associated Press