150 guests at a wedding: Yesodey Hatorah Senior Girls' School, in Stamford Hill, had been covered when they arrived at 21:15 GMT on Thursday.
Guests fled from the strictly Orthodox Charedi Jewish school when the police arrived. The organisers face a £10,000 fine for breaking lockdown rules.
In a statement, the school said its hall had been leased out.
[which is illegal under current lock down Laws]
A spokesman for the school, whose principal Rabbi Avrahom Pinter died in April after contracting coronavirus, said "we had no knowledge that the wedding was taking place".
He added: "We are absolutely horrified about last night's event and condemn it in the strongest possible terms."