Knife crime and school exclusions are likely to be two symptoms of the same underlying problems, according to Ofsted.
The chief of the inspections body, Amanda Spielman, said there was a "harmful narrative" developing that exclusions must cause children to join gangs or carry knives.
But she added the issue was too complex to be reduced to binary arguments, and that schools cannot tackle the issue alone.
In response to an Ofsted report on safeguarding children in London from knife crime, the chief inspector warned that decisions to exclude pupils for bringing knives into London schools do not always take the best interests of that child into account.
She highlighted that some schools are not conducting knife searches, or teaching about knife crime, because they worry about how it will make them appear.
The report says dialogue is missing between local safeguarding partners and schools about the purpose of searching, the impact on staff and pupils and evidence of the impact on knife-carrying.
The research is based on responses from more than 100 secondary schools, colleges and pupil referral units across the capital.
It comes after police chiefs linked rising knife crime to the exclusion and off-rolling of pupils.
The report says schools take an inconsistent approach to dealing with pupils who bring blades into schools.
While Ms Spielman raised concerns about the way some exclusions are carried out.
