You are right that they can be more silent spreaders but this idea just isn't supported.
Schools and SARS-CoV-2 transmission
Based on the data available, in-person learning in schools has not been associated with substantial community transmission. Although national COVID-19 case incidence rates among children and adolescents have risen over time, this trend parallels trends observed among adults.17 Increases in case incidence among school-aged children and school reopenings do not appear to pre-date increases in community transmission.17,32-34
https://www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/transmission_k_12_schools.html