The force said the boy was taken to Wymondham Police Investigation Centre for questioning and was also charged with possession of a knife on school premises.
The boy cannot be named for legal reasons.
The girl, who is a pupil at the school, was taken to hospital with what police described as minor injuries and has since been discharged.
Thorpe St Andrew School, which has more than 1,800 pupils, reopened on Thursday but principal Penny Bignell said children who did not feel ready to return would be supported.
Norfolk Police said it would continue to have officers at the school and in the local area over the coming days to offer reassurance.
