The celebration will be from 9:30 to 11 a.m. at the school at 445 East Charleston Road.
It will include a "welcome reunion," slide show and voluntary sharing time.
Flath, a teacher at Hoover since 1994, was 43. He taught his class as usual Friday (Jan. 28) and was hospitalized during the weekend.
A cause of death has not been determined, but school officials said tests were underway to explore the possibility of meningococcal disease.
Some members of the Hoover community were taking antibiotics as a preventive measure. Representatives from the Santa Clara County Public Health Department held a meeting at the school Monday afternoon.
A health department physician said few members of the Hoover community had the type of contact with Flath that would put them at risk for acquiring the meningococcal bacteria. However, prophylactic antibiotics were offered "out of an abundance of caution," and most parents of Flath's students took them for their children, a health department spokeswoman said.
"Mr. Flath was an uplifting, vibrant teacher who cherished his work here," Hoover School Principal Susanne Scott said in a letter to the Hoover community.
"His sense of humor added to the joy and enthusiasm in his work with students. He will be missed by all of us."
School district staff and Hoover School parents were notified of the death in an e-mail, Assistant Superintendent Scott Bowers said.
Students were told about it, class by class, Monday morning, with counselors on hand, Bowers said.
School psychologists and staff from the grief counseling organization Kara were available to support staff members and students, Scott said.
Meningococcal disease comes from a bacterium that can cause both meningitis, inflammation of brain and spinal cord tissue, or sepsis, a severe infection of the blood, according to Sara Cody, a physician and communicable disease control officer with the health department.
"There are very few people in the Hoover school community who may have had the type of close contact that would put them at risk of acquiring the bacteria," Cody said in a memo to Hoover staff members and parents.
While meningococcal disease is contagious, Cody said, "it is not easily spread from person to person."
Meningococcal bacteria are "not as contagious as things like the common cold or the flu, and they are not spread by casual contact or by simply breathing the air where a person with meningitis has been...
"For those with casual contact, preventive antibiotics are not recommended," Cody said.
The bacteria can live outside the body for only a few minutes, and do not live on environmental surfaces or in the air, she said.
At Monday's meeting with Hoover parents, students and staff, public health officials offered prophylactic antibiotics to those who wanted them.
"Most of the parents did end up taking them for their children," health department spokeswoman Joy Alexiou said.
"But it's extremely unlikely that anybody else will get sick."
Symptoms of meningococcal disease may develop within several hours or over a period of one to two days, and include high fever, severe headache, stiff neck, nausea, vomiting, discomfort when looking into bright lights and mental confusion.
A friend of Flath's, Lewis Weaser, said the teacher, who lived in San Francisco, was socializing with friends Saturday afternoon.
"A group of five of us had dinner plans on Saturday night when he started to mention that he wasn't feeling well," Weaser said in an e-mail Monday.
"That was around 6:30, before dinner."
Weaser said he received word about noon on Sunday that Flath was in the emergency room of Kaiser Hospital in San Francisco.
Flath's family was at his side in the hospital, school officials said.
Rather than sending flowers, those who wish are asked to consider donating to The Companion Animal Rescue Effort at P.O. Box 111474, Campbell, CA, 95011-1474; or the San Francisco SPCA; or the Humane Society of Silicon Valley.