From the States (CDC), updated last month
www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/transmission_k_12_schools.html
SARS-CoV-2 transmission in schools among students, families, teachers, and school staff
With approximately one quarter of teachers at higher risk of serious consequences of COVID-19 because of their underlying medical conditions,53 reasonable concerns have been raised about the occupational risk of SARS-CoV-2 infection for teachers and school staff. Evidence from studies primarily done before vaccine approval for those 12 years of age and older suggests that staff-to-staff transmission is more common than transmission from students to staff, staff to student, or student to student.46, 50, 54 For example, in the large UK study, most outbreak cases were associated with an index case (initial case) in a staff member.46 Therefore, school interventions should include prevention strategies to reduce the transmission potential of staff members. Detection of cases in schools does not necessarily mean that transmission occurred in schools. The majority of cases that are acquired in the community and are brought into a school setting result in limited spread inside schools when multiple layered prevention strategies are in place.38, 55-57
Findings from several studies suggest that SARS-CoV-2 transmission among students is relatively rare, particularly when prevention strategies are in place. An Australian study of 39 COVID-19 cases among 32 students and seven staff traced contacts across 28 schools and six early childhood centers and found only 33 secondary positive cases (28 students and five staff members) out of 3,439 close child contacts and 385 close staff contacts.58, 59 Several contact tracing studies have found limited student-to-student transmission in schools.47, 54, 60, 61 A study of factors associated with SARS-CoV-2 infection among children and adolescents in Mississippi found that school attendance was not associated with a positive SARS-CoV-2 test result. However, close contacts with persons with COVID-19, attending gatherings, and having visitors in the home were associated with SARS-CoV-2 infections among children and adolescents.26 The evidence to date suggests that staff-to-student and student-to-student transmission are not the primary means of exposure to SARS-CoV-2 among infected children. Several studies have also concluded that students are not the primary sources of exposure to SARS-CoV-2 among adults in school setting.47, 54, 59
There is some evidence to indicate that SARS-CoV-2 might spread more easily within high school settings than in elementary school settings.9 For example, researchers in Italy identified and tested nearly all (99.8%) contacts of 1,198 cases in school settings and reported a lower attack rate in elementary schools (one secondary case; 0.38% attack rate) than in middle and high schools (37 secondary cases; 6.46% attack rate).62 This pattern was consistent with findings from a study in New South Wales, Australia, that reported higher attack rates in high schools than in elementary/primary schools.58 The apparent increased risk of SARS-CoV-2 transmission among adolescents may be in part attributable to more social interactions with non-household members outside schools.63 Nonetheless, evidence for greater transmission in middle schools and high schools compared with elementary schools suggests that the former may need to move more quickly to virtual instruction when community transmission is high. Uptake of COVID-19 vaccines in adolescents will likely alter these transmission dynamics.