The BBC reports him as saying
"He said cases were increasing "quite rapidly" among older teenagers and under-21s, but the rate of transmission among school-age children was not changing very much."
But I am not sure what that means. Is he saying that the number of new infections per week is going up rapidly in the first category and not the second? I don't know what rate means here.
Because if he is saying the number of new infections is static weekly for school age children, it's hard to see how that could be the case unless all the infections of children are coming from adults.