Portman, who went to Harvard with Kushner, was on The Late Show with Stephen Colbert, where she was asked if it was true he wasn't a great student.
"That is true," Portman responded with distinct unease.
"Would you care to elaborate?" Colbert followed up, adding that "it's a free country."
Wryly, Portman answered that it was maybe "the last days" of the US being a free country.
"He said in some interview that the friends he has lost because of politics, it's like exfoliating. I was like, 'Okay, I like the spa metaphors,'" she recounted.
"So you're a dead skin cell?" Colbert said.
"Proudly," Portman responded.
Portman and Kushner were good friends during their time at Harvard, and the actress was even invited to Kushner's wedding to Ivanka Trump in 2009. The Kushners were also guests in Portman's own wedding to Benjamin Millepied some three years later.
While Kushner was studying government, Portman was enrolled in the psychology program, serving as the research assistant to Jewish Professor Alan Dershowitz.
Portman has said, however, that over the years she and Kushner grew apart. She even supported Trump's rival, Hillary Clinton, in the 2016 presidential race.
Portman recently came under fire in Israel for refusing to attend the ceremony for the Genesis Prize, which was awarded to her. She explained she did not want to be perceived as showing support to Prime Minister Benjamin Netanyahu, who was due to speak at the ceremony.
In an Instagram post explaining the decision, Portman stressed she did not support the Boycott, Divestment and Sanctions (BDS) movement, which aims to isolate Israel over its treatment of Palestinians.
"Like many Israelis and Jews around the world, I can be critical of the leadership in Israel without wanting to boycott the entire nation," she said. "I treasure my Israeli friends and family, Israeli food, books, art, cinema, and dance."