"If the children are disaffected when they come here, our priority is not always to educate them, but to re-motivate them," she says. "It doesn't matter how many GCSEs they've got, if they can't cope with their lives they'll be useless."
At Trinity, she and her staff are dealing with a typical PRU mixture: primary and secondary children with a wide range of abilities, backgrounds and problems, some short-stay, others not. While nearly 30 per cent are statemented, it is estimated that 80 per cent have special needs in the broader sense of that term.
"We have days when the children can be horrendous," says Marilyn Coombs. "So if one of them comes in effing and blinding, they come to talk to me instead of going into class, and it all comes tumbling out." Today one boy is distraught because his mother has told him never to come home again.
But it's not for nothing that the centre, housed in a pleasant Victorian school building in Trowbridge, is seen by many as a model of good practice. All the primary age and Years 7-9 pupils that come here are re-integrated into mainstream schools after a term or two, and 80 per cent of leavers go on to college.
Many of the children have disastrous home lives. Katrina, aged 16, is a classic example. The child of a blind mother and a criminal father, she's been sexually abused, put into care, and been in trouble with the police. Two years ago she was excluded from school for fighting and bad language.
"The teachers there didn't care, but here you talk about your problems, and get help with your work," she says. Now thoroughly motivated, she's off to college in September, armed with four GCSEs and a range of RSA and AEB qualifications. "I just decided I would change, and try my best."
With seven full-time and three part-time teachers, the centre can offer a reasonably broad curriculum. All the children take GCSE English, science, maths and art, while history, computer studies and photography are also available. Five or six GCSE passes is the norm for most pupils.
The key to success, Marilyn Coombs suggests, is flexibility and co-operation. Of the 156 pupils currently on roll, only 72 actually attend the centre. The rest receive help either through home tuition - usually because of school phobia or a medical problem - or through a teacher from the centre supporting them in school.
The aim is to provide a package for the individual child, depending on need. For a school-phobic child, for example, a home visit may be followed by individual tuition, then a spell at the centre and a gradual introduction into a larger group, before a carefully-timed return to school.
A good and speedy referral system is seen as crucial. The centre is at the hub of an inter-agency network, operating with social services, a behavioural support team, the psychological service, and the child and family guidance service.
Trinity has good relations with local schools, many of which willingly take its children. The exceptions are the grant-maintained schools: "They're less tolerant, they want instant results, and guarantees of good behaviour, which of course you can't give," says Marilyn Coombs.
Like other PRUs, it still has to fight against local misunderstanding and prejudice. "People think kids are just dumped in places like this," says deputy head Peter Grayshun. "In fact there's all the hope in the world for them here."