President Trump’s postelection agreement to pay $25 million appeared to settle the fraud claims arising from his defunct for-profit education venture, Trump University. But a former student is now asking to opt out of the settlement, a move that, if permitted, could put the deal in jeopardy.

Lawyers for the student, Sherri Simpson of Fort Lauderdale, Fla., on Monday asked a federal judge in San Diego to reject the settlement unless former students are given an opportunity to be excluded from the deal so they can sue Mr. Trump individually.

If the judge, Gonzalo Curiel, decides that Ms. Simpson and potentially others should have that chance, legal experts say it could disrupt the settlement because Mr. Trump and his lawyers saw the deal as a way to resolve all of the claims, once and for all, to avoid a trial and distractions to his presidency.

“If even one person could opt out of the settlement and force a trial, that might, in fact, crater the deal,” said Shaun Martin, a professor at the University of San Diego School of Law. “I’m sure Judge Curiel will be aware of that.”