Brennan made his comments during Thursday's graduation ceremony in the school auditorium, which was packed with about 425 friends, family and faculty.
In a world calling out for change, he said, the students in the class of 2004 will provide it.
He said he expects the students - now musicians, athletes and friends - will one day be politicians, business leaders, lawyers and musicians, among other professions.
Brennan, along with Jordan C. Sterner, the salutatorian, and Mia Hornbaker, co-valedictorian, thanked the friends, family and teachers who helped the students reach this point.
"It has been said that 'without teachers the world would stop,' and I agree," Hornbaker said.
At one point, about half of the graduating students walked off stage and stood in front of the stage.
They sang the class song, "Good Riddance (Time of Your Life)" with Brennan directing them and one student playing acoustic guitar.
At the end of the ceremony, one student counted to three and all the students threw their caps into the air.
After the ceremony, students and parents expressed a variety of emotions.
"It is good but sad," Candace Renee Starliper, 18, said.
She said she cried during the ceremony because she already was missing some of her friends.
Joseph Brian Potter, 18, said the ceremony was good but he didn't consider it emotional.
His mother, Denise Potter, disagreed, saying it was an emotional night.
"It was really exciting," Nicholas Eugene Fischer said.
Graduation Day came faster than he expected, he said.