Police were called after the knife attack, which happened at the end of the school lunch break.
Police say two teenagers got into difficulty swimming below a weir close to Darley Abbey Mills.
Darley Abbey Mills Bridge closed in 2022, with a temporary replacement also shut in December 2025.
Derby City Council leaders have agreed to take ownership of the "extremely vulnerable" structure.
A bridge in Derby is due to stay closed for a further 12 months because of safety concerns.
The council says they are frustrated for businesses but public safety "must come first".
The bridge was closed after the city council identified erosion to the River Derwent's riverbank.
Derby City Council says the crossing has been shut due to erosion to the River Derwent riverbank.
Hundreds of drivers have been fined for using the gate during restricted times.
