The WHO and the American Academy of Pediatrics. But the CDC says 6ft.
I always find numbers like that interesting.... Like 6ft is good, but 5 feet? You're gonna die. 10 people in a group, you're golden, 11 people? Waaaay too many.
Reminds me of the Monty Python scene about counting to 3.
Anyway, here's the info:
RICHMOND, Va. (WDBJ) - Virginia education leaders have updated their guidance for reopening the commonwealth’s schools this fall, with schools having been closed in the spring because of COVID-19 concerns.

The new suggestions were sent to superintendents and local school leaders, who have the ultimate responsibility, according to the Virginia Department of Education, for deciding the best plans for their individual districts.

Click here to read the full letter of guidance sent Monday.

Suggestions from the Centers for Disease Control and Prevention, World Health Organization and American Academy of Pediatrics are included in the new guidance, which includes some key recommendations:

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 CDC advises that individuals maintain six feet of distance to reduce the risk of COVID-19 transmission. The World Health Organization (WHO) advises that schools maintain a distance of at least one meter (approx. three feet) between everyone present at school, and is monitoring ongoing research. Additionally, the American Academy of Pediatrics (AAP) says spacing as close as three feet may have similar benefits if students wear cloth face coverings and do not have symptoms of illness. Physical distancing is not limited to distance between children; physical distancing between adults is a key mitigation measure. It will be important to continue to monitor the community context of COVID-19 prevalence into the fall and winter. In areas where the community transmission of COVID-19 is more substantial, distancing of at least 6 feet will need to be strongly considered; this guidance may be subject to change as we learn more.

 Therefore, in school settings, schools are encouraged to aim for six feet of physical distance to the greatest extent possible however, if six feet of distance is not feasible (inclusive of buildings and school buses), schools should implement a combination of face coverings and a minimum of three feet distance between everyone present.

 Physical distance should be created between children on school buses when possible (e.g. seat children one per seat, every other row and/or staggered, aisles and windows) limiting capacity as needed to optimize distance between passengers. If three to six feet of distance can not be maintained, wearing of face coverings is strongly encouraged and may help reduce disease transmission. Children (such as siblings) living together may sit together on the bus, and assign seating where possible. If possible given the age of students, weather conditions, etc., consider opening windows to improve ventilation.

 In addition to physical distancing described above, other physical distancing precautions should include, but are not limited to: o Restrict mixing classes/groups of students. o Close communal spaces. o Limited athletics and extracurricular activities. o Limit outdoor activities/recess to 50 people, with a priority on physical distancing and restricting mixing of classrooms. o No gatherings (assemblies, graduations, etc) of more than 50 people (indoor or outdoor). Indoor gatherings should be held only as necessary, and be limited in duration.