In nearly four decades at McDonogh, Mr. Seal taught lower school science and upper school psychology and history, guided the senior class, and coached soccer, lacrosse and wrestling. He was inducted into the McDonogh Athletic Hall of Fame in 1999 and received the Alumni Association's Distinguished Service Award in 2007.
"Bill had high standards, high expectations and great compassion for students. He could see the world from the perspective of a student, and he recognized the human foibles and flaws that are found in all of us," said William "Bill" Mules, a former McDonogh headmaster who lives in Baltimore. "He had a sense of humor that lifted the spirits of all those around him, colleagues and students alike. He had the ability to get inside a kid's skin."
Born in Baltimore and raised in Hamilton and Reisterstown, he was a 1965 graduate of Franklin High School, where he wrestled and played soccer and baseball. He earned a degree in the history of psychology from the University of Baltimore. He later earned a master's degree in counseling from what is now Loyola University Maryland.
Mr. Seal initially worked as a special education teacher in Carroll County public schools. He was enrolled in law school when he accepted a position in 1972 as a science and woodshop teacher in McDonogh's lower school, where he also established a physical education program.
"He was the first faculty member I met on the first day of school as I got off a bus," said John O. "Jack" Mitchell IV of Lutherville, who is a school trustee. "He embraced all levels of the school and was active in all its facets."
In 1986, Mr. Seal became the school's associate dean of students, an assignment wherein he and his wife, who were living on campus, assumed the role of head house parents.
"There was a time when there was a question whether the boarding program would continue," said Kennedy Waller, a former student who is now a faculty member. "He helped save it by bringing innovations to it."
Mr. Seal later became the senior class dean and also taught psychology and world history.
"Bill had a wonderful spirit about him. He was one of the most cheerful people. He was never down. He loved the work he did and enjoyed every minute he spent with young people," said Ceil Millar, a McDonogh faculty member and former upper school head. "He had a way with working with kids struggling with issues in their lives. He created a safe place where they could open up."
Mr. Seal coached his final soccer game in the fall of 2010. He was then an assistant varsity coach and he had a conference championship game against the Gilman School, where his son, Jonathan Seal, was the head varsity soccer coach. In that contest, Gilman beat McDonogh. Father and son sat on opposing benches.
"He said that if he were to lose his final game of his career, he would rather have it be to his son," said Jonathan Seal, who lives in Baltimore.
Friends said that Mr. Seal enjoyed puns and liked waging pun contests with his colleagues.
"We will miss his sense of humor and his laugh, his sweater vests, the occasional bow tie, and even the needlepoint glasses case, but most of all we will miss the school person he has always been," said McDonogh Headmaster Charlie Britton in remarks made when Mr. Seal retired in 2011.
A memorial service will be held at 11 a.m. Saturday at Tagart Memorial Chapel, 8600 McDonogh Road.
In addition to his son, survivors include his wife of 45 years, the former Cheryl Working; his mother, Ann Naumann Seal of Timonium; a brother, David Charles Seal of Sykesville; and four sisters, Karen Dzialowski of Reisterstown, Lauren Unkle of Shrewsbury, Pa., and Jackie Levin and Susan Pontuti, both of Monkton.