They were all part of a group of 112 students from two schools who made the annual trip to the nation's capital and then returned from the trip on Sunday.
The total number of students infected with COVID19 is 56, or 45%.
"We call that the attack rate that’s a relatively high attack rate," said Dr. Matthew Willis with Marin Health & Human Services.
KTVU confirmed that one of the schools affected is Mill Valley Middle.
"We do know that some children from Mill Valley Middle School attended the Worldstrides trip to Washington D.C. this past week, while others traveled with family, or stayed in the area. This Worldstrides trip was not a school event," read part of a statement from Dr, Kimberly Berman, Mill Valley Unified superintendent.
County health officials said they had tested positive before returning to school the next day. Marin County Public Health issued an advisory to all middle schools offering guidance on Monday.
Public health officials said all the infected students have reported mild or no symptoms and none are hospitalized.
A total of 90% of Marin County 8th graders are fully vaccinated with over half of them having received their booster shot, officials said.
Washington D.C. is currently undergoing a high-profile coronavirus surge with Rep. Rashida Tlaib announcing on Monday that she is among a wave of Washington leaders who have tested positive.
Marin County Public Health recommended that students and staff who participated in the Washington field trip may return to class unless they have tested positive for COVID or are symptomatic. They should also test for COVID prior to returning to class and continue testing for five days after their return or sooner if they experience symptoms.
Participants in the field trip should wear a mask indoors in school and in public for five days upon returning from the trip if anyone within the group they traveled with tested positive for COVID.