It looks like in November/December the rate among grades 7-11 was almost 3x the rate of those ages 35-39.
https://twitter.com/c_drosten/status/1347544994941431808?s=2...
https://www.ons.gov.uk/peoplepopulationandcommunity/healthan...
Also, most schools were closed from the beginning of January to March.
https://www.nytimes.com/2021/01/04/world/europe/uk-lockdown.... https://www.bbc.com/news/education-51643556