There had been no signs of the boys in the 48-degree water since the incident Saturday.
"The ocean is unpredictable, and there's no way to know where they may surface," Coast Guard Petty Officer Michael Burgess said.
The victims were part of a group of 18 students and six adults who had traveled from William Aberhart High School in Calgary, Alberta, to hike along the isolated coast 220 miles north of San Francisco.
Barbara Clement, 45, was caught by a wave and pulled out to sea. The two 17-year-old students, David Elton and Brodie Macdonald, jumped into the water to try to rescue her.
A few moments later, teacher Martin Poirier, 37, and a third student, Jordan Nixon, 17, tried to rescue the boys and Clement.
All four were captured by the powerful rip tide. Clement drowned, while Poirier and Nixon were dragged out of the pounding waves by fishermen. Macdonald and Elton were presumed dead.
Locals said the stretch of beach is extremely dangerous because of the strong undertow, rip currents and unpredictable waves.
"There's been vehicles washed off that beach," said Pete Winkler, who was fishing for salmon when he heard the Coast Guard's call for help and joined in the search.