Under the new policy, the government would begin to calculate what is called “unlawful presence” from the date that visa holders’ purpose in the country has expired, such as the end of their studies. Under the current policy, put in place 20 years ago, that calculation started once the government discovered the violation.

The new policy would also be less forgiving. Visa holders found to be in the country illegally for more than 180 days would be barred from re-entry for three or 10 years, depending on the length of their overstay. The current policy usually allows students to go back to their country and apply for a new visa, or update their visa status and return.

The harsh penalties are especially jarring because small mistakes could now carry big consequences, said Angelo A. Paparelli, an immigration lawyer at Seyfarth Shaw LLP, a firm based in Los Angeles.

“For immigration attorneys, because time is of the essence, if the three- or 10-year bar is triggered, there’s not much you can do,” he said.

It is common for students to fall out of compliance while awaiting a new visa or transitioning to a new one, Mr. Paparelli said. Students who use visas to stay and work after completing their degrees are particularly vulnerable. Determining whether their jobs meet certain requirements to stay is subjective. Their violations could be applied retroactively, and subject them to an automatic ban, he said.

“The effect of this change will be felt by businesses,” he added. “It will foreclose what have been standard approaches to transitioning from student to worker, whether that’s on an H1-B or some other work visa category, or the transition to permanent residence.”

Administrators who work with international students worry that they will be punished for life events that domestic students are usually supported through. Many students fall out of status because of extenuating circumstances, such as temporarily dropping below a full course load because of mental health issues, leaving school temporarily for a family emergency or picking up a part-time job to help with family finances.