The point David Anderson is trying to make is that the decision to exclude a pupil from school is not reached lightly. The exclusion rate at St Chad's is high: last year there were 90 short-term and six permanent exclusions; this year it stands at 41 short-term and five permanent, two of them reinstated - but it is coming down.
"We have to be seen to be trying every possible strategy in order to avoid a permanent exclusion, which is very upsetting to a pupil's future education," he says. "I will only do it if I believe that I have done everything I can possibly do for the child."
St Chad's has a number of strategies for improving pupils' behaviour, the first of which is by rewarding them when they do well. Pens, trips to a burger bar and cinema tickets are on offer.
But when these incentives fail, there is a range of sanctions to fall back on, from short lunchtime detentions to a warning letter being sent home.
After three of these letters, the parents are asked into school to discuss the situation. The second time this happens, a short exclusion of up to five days may be imposed, and on the third occasion the school's governors discuss the case. Sometimes, one of them will offer support, dropping in to see the pupil and the parents and monitoring the child's behaviour.
Not many pupils get to this stage - only about two every half-term - but for those who do, an exclusion of 10 days is considered. As a last resort there is the possibility of permanent exclusion.
There are also internal exclusions in which the child works outside the head's office or spends a day or half a day with a head of year. There used to be a space set aside for this, but it was dropped because "Room 29" too often became a focus for troublemakers.
While all this is going on, the child is also assessed to see if she or he has learning problems or emotional difficulties.
Mr Anderson, who has been head of St Chad's for eight years, says many of his staff believe he should permanently exclude more pupils. But he knows he has a responsibility to balance the teacher's need for a safe working environment with the pupil's need for an education.
"When pupils are excluded from one school and move to another, in my experience it seldom works out; the problems continue. It isn't a solution," he says.