The 600-pupil Arts and Media school in Islington had to close last week as 40 pupils ran amok following a row between girls from different ethnic groups.
The school opened on the site of the George Orwell school, which was judged to be failing by the Office for Standards in Education.
The Arts and Media school brought in security patrols during summer refurbishment work to deter trespassers. They have been kept on by the principal, Torsten Friedag, although he hopes to dispense with them eventually.
Every pupil must show a security pass and has to be ticked off an initial register by a guard before being able to enter the school.
Mr Friedag stressed that he wanted to work with the community but first he had to make the school safe.
Andrew Adonis, education adviser to the Prime Minister, is a governor at the school.