Among Van Dyke's high school classmates in Danville were Donald O'Connor and Bobby Short, both of whom would go on to successful careers as entertainers. One of his closest friends was a cousin of Gene Hackman, the future actor, who also lived in Danville in those years. Van Dyke's mother's family was very religious, and for a brief period in his youth, he considered a career in ministry, although a drama class in high school convinced him that his true calling was as a professional entertainer. In his autobiography, he wrote, "I suppose that I never completely gave up my childhood idea of being a minister. Only the medium and the message changed. I have still endeavored to touch people's souls, to raise their spirits and put smiles on their faces." Even after the launch of his career as an entertainer, he taught Sunday school in the Presbyterian Church, where he was an elder, and he continued to read such theologians as Buber, Tillich, and Bonhoeffer, who helped explain in practical terms the relevance of religion in everyday life.
Van Dyke left high school in 1944, his senior year, intending to join the United States Army Air Forces for pilot training during World War II. Denied enlistment several times for being underweight, he was eventually accepted for service as a radio announcer before transferring to the Special Services and entertaining troops in the continental United States. He received his high school diploma in 2004 at the age of 78.