A hilarious and self-deprecating keynote, ending with an off-color song, marked the occasion of the 174th Commencement as Brad Hall ’75 returned to his alma mater.

The actor, writer, and director began his remarks under the tent on the Quad by noting that he had been having a recurring nightmare about returning to the Williston Northampton School to give an oral report—dressed as Supreme Court Justice Ruth Bader Ginsburg.

Pausing for a moment, so the audience could take in his black, formal robes, Mr. Hall quipped, “Thank you for helping my most horrible dreams to come true.”

After comparing his own experience at Williston to living in a teepee during the ice age and hunting mastodons, Mr. Hall noted that the 139 seniors “seem to really appreciate the gifts you’ve been given.”

“What am I supposed to tell you guys about the future?” he asked. “You guys literally are the future.”

He did, however, wish the Class of 2015 both tremendous success and fabulous failures—particularly the humility and compassion that could be learned through the latter.

“There are times when a little suffering, a little hardship, and a little failure have value,” he said. “Even those of us lucky enough to receive a Williston education will someday fail.”

In a wonderfully wry finale, that drew a standing ovation from the senior class, Mr. Hall concluded his speech with an off-color song he had written for the occasion, called “Don’t be an a***hole.”

“Don’t be a slimeball, a dirtbag, or weasel,” he sang. “Put your phone down once in awhile. Say hello to people.”

“I’ve been to a lot of Commencements in my 30 years—that’s professionally and then as a student—and I guarantee you’ll remember that one for the rest of your life,” laughed Head of School Robert W. Hill III as he took the stage after Mr. Hall.

As part of the Commencement ceremony, Emily Grussing was awarded the Sarah B. Whitaker Prize (White Blazer) honoring the top young woman in the class, and Cade Zawacki received the Archibald V. Galbraith Prize and was named the class valedictorian.

Mr. Zawacki returned to the stage as the senior class speaker and explained that his address would be unusual, since it was the result of informal polls he had conducted among his peers over the course of the past two years.

As a result, he offered Disney movie quotes (“I was hiding under your porch because I love you”), a few jokes from a 99 cent book, and compared Williston to the warm, homey comfort of waking up to the smell of chocolate chip pancakes.

“Williston is those pancakes,” Mr. Zawacki said. “Never, not even for a moment, take for granted what Williston has done for you.”