Charles Spencer Posts Rare Childhood Photo with Princess Diana amid New Memoir: 'First Day of School'

The 9th Earl Spencer reflected on a tender milestone with his sister by his side

Charles Spencer is looking back at his school days with his sister Princess Diana amid the release of his new memoir, A Very Private School.

On April 3, the 9th Earl Spencer shared a childhood photo with Diana dated to 1968. The shot showed the brother and sister matching in red uniforms, which Spencer said was taken on a milestone day in his education at age 4.

"My first day of school, in September 1968: my father took this photograph of me and my sister, Diana, just before he drove us to Silfield, a really lovely primary school in King’s Lynn, Norfolk. The headmistress was Miss Jean Lowe, a warm and thoughtful lady who loved her boys and girls," Charles captioned the photo on Instagram. "I was there till 1972, when I headed off to the place I call - in my memoir - A Very Private School."

Related: Charles Spencer on Whether He Told Sister Princess Diana About His Boarding School Abuse (Exclusive)

He was referring to Maidwell Hall, the all-boys boarding school that Spencer, 59, attended from age 8 to 13. The historian confronts the childhood trauma he suffered there in A Very Private School, revealing for the first time that he was a victim of physical, verbal and sexual abuse at the school.

In the new memoir, which was exclusively excerpted by PEOPLE and published on March 12, Spencer revisits the harrowing five years at an elite institution in the English countryside that left its young pupils vulnerable to predatory adults.

Related: Charles Spencer Honors Sister Princess Diana on All Souls' Day with 'Deeply Poignant Photograph'

"I had always felt that I had dealt with this chapter of my life and had put it to bed," Spencer tells PEOPLE. "But talking to classmates and hearing their experiences was very triggering for me. I'd be very struck by the horror of what they had shared. And that was the hardest bit."

Though the 9th Earl Spencer was exceedingly close with Diana in childhood, he says that neither she nor his other sisters knew what he suffered within Maidwell’s halls.

"I don’t remember us ever really discussing what we were going through. She went to a very gentle place, I believe," he says of Diana’s boarding school years. "We would reconnect for the school holidays, and I don’t remember ever talking about it with her."

When Spencer finally told older sisters, Sarah and Jane, 18 months ago, "they were stunned and appalled" — while Diana never knew.

"From when I was a baby, Diana used to look after me," he says.

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After putting pen to paper, Spencer worries that in sharing his story, some will judge his candor as a plea for pity from those born into tremendous wealth and privilege.

"My biggest fear is that people might think, 'Poor little rich boys. What have they got to complain about?' It’s not about the wealth. It’s about vulnerability as children in any social class. My biggest hope is that it genuinely will help people," he tells PEOPLE.

If you suspect child abuse, call the Childhelp National Child Abuse Hotline at 1-800-4-A-Child or 1-800-422-4453, or go to www.childhelp.org. All calls are toll-free and confidential. The hotline is available 24/7 in more than 170 languages.

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