(KRON) -- In-person classes at a Catholic high school in San Francisco have been canceled due to a tuberculosis outbreak. In a letter sent to the community, Archbishop Riordan High School President Tim Reardon said classes on campus will be canceled on Friday as the school prepares for "modified ins truction." Next week, Riordan's classes will be "asynchronous," which means there will be no live instruction. Students will be assigned homework for them to complete off campus. Reardon said hybrid learning will begin the following week, starting on Feb. 9. As of Thursday evening, there have been three active TB cases confirmed, according to the San Francisco Department of Public Health (SFDPH). The first case on campus was reported in November. The risk of TB transmission at the school is currently considered "moderate," officials said. Bay Area rock climbing gym to ‘stand with the people,’ close for economic blackout The Archbishop Riordan High School band readies to play on the court before the varsity basketball team's game against St. Ignatius on the soon to be dedicated Restani Court, honoring Kevin Restani, a 1970 Riordan graduate who starred at USF and in the NBA. Photographed in San Francisco, Calif., on Tuesday, January 23, 2018. (Photo by Scott Strazzante/San Francisco Chronicle via Getty Images) SFDPH was made aware of this outbreak and is requiring all Riordan students and staff to complete TB testing, the agency said. Starting Feb. 20, only staff and students who've completed testing may attend in-person indoor activities on campus. Students who haven't completed the required testing by Feb. 20 will not have an online option to complete any coursework. Reardon said those students will need to use their parent-excused absences until they complete testing for TB. TB is a serious but treatable disease, health experts say. It is transmissible through the air when coughing, laughing or singing. Symptoms include fever, coughing up blood and loss of appetite. ...read more read less