Before that, he had praised Freddie for his letter telling a member of staff: "Lots of people write letters but Freddie's was a really well-written letter. I promised I would come."
Rowley Regis, Sandwell, to meet pupil Freddie Hadley, 12, after receiving a letter last year inviting him to see the mental health initiatives at his school.
Prince William attended the school as a surprise for pupil Freddie who invited him (Image: Getty)
Today's visit marked the first time that William has made a royal visit since news of the Princess of Wales's cancer diagnosis and treatment was publicly announced last month.
And to his surprise, the Prince of Wales replied, saying: "Good afternoon Freddie, I'm so sorry Catherine and I can't be with you and the rest of the students at St Michael's today.
"Tackling mental health challenges and stigmas head-on is so important, please keep up this important work."
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Following the letter, William asked his team to secretly set up a visit to the school next time he was planning public engagements in the West Midlands.
The schoolboy and his classmates could be seen visibly shocked as the royal wandered into their classroom to undertake the first of three visits in the area.
School staff were able to keep the visit a secret by telling Freddie he was instead being visited by the Lord Lieutenant of the West Midlands.
And Freddie still did not realise who was actually visiting him when he saw the prince's motorcade approaching the school, until he saw William step out of the car.
William told Freddie: "So this is the young lad who started it all off. Very nice to see you, thank you for your letter."
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