Gastroenteritis is highly contagious which is why school administrators are cancelling classes for Wednesday. The illness also carried into the basketball games on Tuesday night when some students became ill.
More than 1,400 students attend St. Ignatius, 50 students called in sick on Tuesday, and another 150 went home after not feeling well.
"People were throwing up everywhere, coughing everywhere," said Carina Maysenhalder, a freshman.
"I came out of class and there was about 20 students sitting right in front of the dean's office with trash cans everywhere and I actually saw some kids throwing up. It was not a sight to see," said Josie Chan, a freshman.
School is cancelled on Wednesday while cleaning crews sanitize any surface students might come in contact with such as desks, lockers, door handles.
San Francisco city health officials believe this is not influenza, but an outbreak of viral gastroenteritis, also known as stomach flu. It is very infectious and can be spread when people don't wash their hands after using the restroom or after getting sick. The germs can linger for days if not treated.
"In order for us to properly clean the facilities, so that this doesn't spread to anybody else, we're taking the precaution to close down school so we can do a very through and deep cleaning," said Patrick Ruff, the school principal.
A reception for applicants who want to attend St. Ignatius was cancelled Tuesday night, but not the scheduled girls' basketball games against Sacred Heart Cathedral Preparatory.
"During the last few moments of the game, one of the kids from S.I. started vomiting on the court and she had been vomiting throughout the whole game," said Ken Smith, a Sacred Heart parent.
The principal and the San Francisco health officer said there was no need to cancel the games. One parent commended the school for cancelling class tomorrow.
"Everybody knows kids can get sick, but I'm appreciative that the faculty and administration are sensitive to everyone's needs," said Mike Maysenhalder, a parent.
Students who didn't get sick are taking precautions.
"Always washing my hands, not touching my face like after I touch anything that's been touched by other people. I have hand sanitizer with me everywhere," said Chan.
A health official for the city says some people may have been exposed to the bug by Tuesday night's sickness. Health officials will be on campus Wednesday. In the meantime, the school is telling students to stay home for 72 hours after vomiting has stopped or until their fevers go away.
You might recall that in 2009, hundreds of U.S. schools closed because of the swine flu. More than a dozen were closed in the Bay Area, some for more than a week.