Take two secondary schools. They are both in deprived areas. They both have challenging pupils - many from poor families, many with special needs. Yet in one of them the staff are confident, communicative professionals. They like their head. In the other, an atmosphere of isolation and cynicism rules. The teachers do not trust the head.
Now compare the number of children they permanently excluded over four years: the unhappy school expelled 33, the happy one zero. Is there any connection?
Elle Rustique-Forrester certainly thinks so. The American researcher, based at Stanford university, studied four similar secondaries in England. She concluded that their rate of exclusion directly reflected the ethos of the schools and their respective styles of leadership.
The heads more likely to exclude are those more vulnerable to outside pressures, to league-table rankings and the knock of the inspector at the door. They are less likely to have systems to support teachers, and are not focusing on pupils' individual needs. The staff are miserable and the children are paying the price.
For some, that price is very high. Everyone knows that exclusion is a last resort, a desperate measure seized on by heads and teachers worried about their other pupils - and their results.
Exclusion has always been controversial. Too many African-Caribbean children, youngsters from poor families and those with special needs are shown the door. Once out, their fate is uncertain. Only a quarter of local authorities manage to provide them with the full-time education to which they are technically entitled, putting the rest at risk of falling by the wayside. So the more schools that can behave like Langdon in East Ham, east London, the better.
Most years, the 1,800-pupil secondary permanently excludes no one, and the figure is never more than one. This is unusual in a country that saw expulsions soar by 400 per cent after the Conservatives unleashed the national curriculum, testing and league tables and the subsequent obsession with comparing and ranking schools.
While numbers dropped back under Labour, which had to abandon targets to cut exclusions, around 9,500 children are still shown the door every year - more than three times the 19901 figure.
Langdon's headteacher Vanessa Wiseman, who has been in the job for 12 years, says: "When I started as a head and had to do more exclusions, I realised that it was a very negative use of time. We would rather have used that time to support and establish the ethos of the school."
Backed by the governors, that ethos is now firmly in place. It is an ethos of openness, honesty and warmth - "though we do have our moments", she says.
Dr Rustique-Forrester, who studied Langdon, talks of a positive staff culture, created by "distributing" leadership throughout the school and developing systems to support staff and students. Staff communication, the relationships between teachers and pupils, and understanding children's individual needs are priorities.
Langdon is organised, says Ms Wiseman, so that there is shared leadership across the school, which is one of the biggest 11 to 16 secondaries in the country.
At many levels within senior and middle management and within the pastoral and support staff, teams are set up with "strong leadership committed to inclusion".
And communication is crucial.
"Like lots of schools we have a good number of challenging young people," she says. "Here, we can have those discussions that allow staff to bring up concerns. They can say 'I am having difficulty here' without immediately being treated as if they are not very good at the job.
"I remember when I was a young teacher having problems with a pupil and another teacher would say to me, 'Well, I don't have any difficulty with them - they were all right for me.' That sentence just condemns you."
Not surprisingly, a key role for Langdon's senior staff is to support new teachers. Ms Wiseman and the head of the other low-excluding school that Dr Rustique-Forrester studied believe in praising people, in working as a team, in sharing decision-making and delegating responsibility. And they are rewarded with staff who are prepared to be innovative and flexible.
By contrast, the schools that excluded more pupils "created a far more sobering picture of what occurs when teachers feel unsupported by the schools' leadership and senior management team, are isolated through lack of communication and collaboration with colleagues, and have few structures and systems to rely upon for supporting pupils' individual needs," says Dr Rustique-Forrester, in a paper she presented to the British Educational Research Association in September.
The head of one such school explained that she was not a popular appointment. Another, under some pressure to raise standards, said that there was simply no time for talking. In these schools, even if information is gathered about a difficult child, it doesn't necessarily translate into action. Pupils might be sent to the student-support centre with little thought about how to bring them back.
Rather than nurturing in-house talent, the heads tended to look outside for specialist help, but the specialists did not usually interact with, or see themselves as part of, the staff. As for meeting pupils' needs, in these high-excluding schools, says Dr Rustique-Forrester, the emphasis is on fitting the child into the curriculum, not vice versa. At Langdon, though, the same positive ethos that supports staff also operates for pupils.
"They all have different people that they can go to, confide in, and get support from, which is particularly important in a big school," says Ms Wiseman.
"It makes them want to be part of the place. It is their school and they do feel proud of it - in the best sense - and they do want to do things for it."
Dr Rustique-Forrester says that low-excluding schools such as Langdon look inwards rather than outwards, focusing not on government definitions of success, such as league-table rankings, but on internal development of the culture and structures they need.
This inward-outward difference is reflected in how they deal with children at risk. Both of the secondaries with low rates of exclusion have created their own multi-layered support systems.
To start with, the teachers talk to each other about the pupils, writing down and sharing information about problems. Someone takes responsibility for checking on a child's progress. Then there is the learning-support unit, the behaviour-support teachers, the mentors, the school inclusion workers and so on. All work together with the aim of keeping children, if humanly possible, in school.
The lessons of Langdon are highly relevant as Labour shows little sign of easing the pressure on schools. Some of its new academies, in the spotlight to achieve, seem to have turned to exclusion as a way of raising standards.
The King's academy in Middlesbrough permanently excluded 28 pupils in 2003-4, more than 10 times the average for secondary schools in England. The town's other academy, Unity City, also expelled an above-average 14 pupils.
Ms Wiseman says: "There is always a tension for everyone between the issue of results and ethos. I never like to generalise, but we would say that our improvement academically is supported by having an inclusive ethos. It has had knock-on benefits for both our academic achievement and the behaviour of all our students.
"We are trying to create a community that everyone feels a part of. It sounds like a cliche, but that's what we're about."