Schools must monitor all exclusions, including those from a lesson, and use the information to plot whether the same teachers or pupils were involved, said Sandra Mohamed, policy officer for the pressure group Local Schools Information.
She was addressing a conference on supporting African-Caribbean boys at school, organised by the National Children's Bureau.
Black pupils are between three and six times more likely to be excluded than white children. The Office for Standards in Education found last November that 25.7 per cent of all exclusions are from minority groups, and that two-thirds of those excluded are below average ability.
Ms Mohamed said: "Primary schools should have a clear policy on not (repeat not) to exclude," she said.
"It leads to a cycle which is hard to break - it's where children begin to get lost in the system."