ASPEN, Colo. -- The Aspen School District is asking 37 children to stay home until May 11.

In a letter to parents, the superintendent's office said it is asking two groups of students to stay home.

One group of 23 sixth-grade students returned last Friday from a coordinated service trip to San Diego, Calif. Parents of those children are being asked to keep the kids out of school until May 11 because of the high incidence of swine flu cases in the San Diego region.

The letter said, "This will take those children beyond the 7 day incubation period and assist us with insuring the safety of other children on our campus."

Any families returning from Mexico are also being asked to hold their children out of school for the seven day period following their return. The superintendent's letter said there were about 14 children returning from Mexico last weekend. The school district is contacting each of those families and asking them to keep their children home until May 11.

Another group of students is returning from New Mexico on a service trip. The district letter said, "In our conversations with county health officials, we have determined that they have been somewhat isolated and in essentially a controlled quarantine." The students in New Mexico were on close contact with students from Mexico, but none of the students from Aspen or Mexico have showed any signs of illness. Those children were allowed to return to school Monday.

The Aspen school district also said as a precautionary measure, it has ramped up all cleaning efforts across the school district and increased the availability of hand sanitizers across the campus.

Read the letter from the Aspen school district's Web site.