FARGO, N.D. —  Hundreds of students answered an urgent call for sandbaggers Monday to protect homes from the rising Red River, expected to crest about 20 feet above flood stage this week.

The response was so overwhelming, district officials had to limit the number of students excused from class for sandbag duty to 300, said Fargo School District spokesman Lowell Wolff.

"We think we're on top of the situation," said Mayor Bruce Furness, adding that the city should be in good shape as long as it doesn't get more rain.

The Red River, which runs north along the North Dakota-Minnesota line, has been swollen by melting snow and storms that have brought downpours. The National Weather Service expects it to crest Wednesday in the Fargo area at about 37.5 feet; flood stage is 18 feet.

"This has been a very dramatic rise. We have not seen a 5-foot increase [every day] for two or three days — ever," city Public Works Director Dennis Walaker said.

If the river hits 37.5 feet at Fargo, it would match the level that caused the area's second-worst flood of the century, in 1969, Walaker said. The worst was in 1997 when the river hit 39.5 feet.

In the years since those devastating floods, however, the city has bought up flood-prone land, removed homes and built new levees. In the area the students were filling sandbags to protect, about 75 homes were in danger of flooding.

The Red River already crested at Wahpeton, about 50 miles south of Fargo, after reaching 15 feet, about 5 feet above flood stage, the weather service said.

Upstream, it was expected a crest Thursday at 47 feet near Grand Forks, 7 feet above flood stage.

"At 47 or 48 feet, we have no concern," said Jim Campbell, Grand Forks County emergency manager.

One of three bridges between Grand Forks and East Grand Forks, Minn., was closed Sunday, and another was being shut down Monday, leaving only one open between the two cities. Minnesota Gov. Tim Pawlenty also authorized the mobilization of 135 National Guard soldiers to help with dike patrols, security and traffic control.

During the 1997 flood, some 50,000 people were evacuated in Grand Forks and East Grand Forks as the river crested 26 feet above flood stage.