The Hangover superstar returned to his alma mater, Germantown Academy in Fort Washington, Pennsylvania, to catch up with his friends from the class of 1993 and visit his favourite teachers.
He arrived with his friend and fellow 93 classmate Brian Klugman, who stars on the Fox series Bones.
Bradley roamed the halls with his former history teacher Kendall Mattern, and later returned with Brian to attend the reunion party at MaGerks Pub & Grill.
"He has come back to GA a couple of times to talk to students," a school official told People. "He loves seeing his old teachers and enjoys reminiscing with them.
"When he says he's going to do something for the school, he does it. He's a big supporter and proud to be a part of this school. So it was no surprise that he came to the reunion."
A source confirmed to E! that the unflappable actor didn't have any security, was "very casual" and a "super nice guy" who seemed "happy to be there".
The 38-year-old took photos with his old friends and former peers, who treated him "as one of their old classmates".
It seems Bradley enjoyed the reunion so much that he and Brian decided to stick around until the next day to cheer on Germantown Academy's football team as they took on arch rivals Penn Charter.
After losing 12 years in a row, GA manage to beat the opposition — and students joked that it was Bradley's presence that helped break the losing streak.
"He hung out as long as he could at the game. He loved being back at the school," a source told People. "He took his time signing autographs and said hello to lots of people.
"He is very modest. I think he has a hard time understanding why people follow him round campus. It's genuine and not a false modesty. He's a really good guy."