TEACHERS with mental problems or criminal records are able to work in schools because the Education Department makes no such checks on their backgrounds. Last week a court was told that a teacher, who admitted sexually assaulting a schoolgirl, had received months of counselling for his ''problem'' before committing the offence. The 35-year-old teacher is now awaiting sentencing for his abuse of the 15-year-old retarded pupil. During the hearing he was described as suffering from a mental problem which ''prevented him from controlling his sexual urges''. Prior to the offence he had undergone eight months of counselling with a social worker. His lawyer said: ''He does have a problem. He knew he had a problem and it was something that came to him from time to time.'' So how did this man end up in a position that enabled him to remove a girl from another teacher's class in order to abuse her? Teachers in charge of mentally handicapped or maladjusted children are generally screened in respect of their qualifications and not their personal histories, according to the Department of Education. And, say Social Welfare Department sources, school principals may provide ''glowing references'' for teachers they suspect to be a risk, in order to quietly remove them from their own schools. A spokeswoman for the Department of Education refused to discuss the incident, describing it as ''an isolated case''. But she admitted that, other than written references, no checks were likely to be made on a teacher's character providing he or she had sufficient teaching qualifications. ''Teachers in special schools are all qualified teachers. In addition to their teaching qualifications they receive training in special education. ''The training is not purely academic - it includes practicals and teaching in the classroom. They are also judged on some personal characteristics, such as whether they are able to communicate with the children. It's quite intensive,'' the spokeswoman said. But no checks are made as to whether the teacher has a criminal record or has undergone treatment for conditions that may affect the safety or well-being of his charges. Legislation making the disclosure of certain types of psychiatric conditions compulsory for teachers may go some way to reducing the risk of abuse. In the case of mentally handicapped children, checks like this are especially necessary, according to Dr Patricia Ip, vice chairwoman of the organisation Against Child Abuse. She says mentally handicapped or disadvantaged children are at an increased risk of abuse from teachers because they are both more vulnerable and less likely to tell another adult than other children. ''I think it's probably going on more than we think,'' she said. Wherever there is a teacher-pupil relationship, it is impossible to be ''absolutely sure'' abuse is not occurring, Dr Ip said, but added that better communication within schools and extra supervision of teachers should reduce the risk. ''The schools themselves have to look at procedures and see how the teachers and children interact. There should also be a review of our internal procedures to minimise the occasions where this kind of thing could happen,'' she said. There are 40 schools for mentally handicapped children in the territory, and a further seven schools for ''maladjusted and socially deprived children''. Teachers employed at these schools undergo normal teacher training. They are then trained locally through a one-year, full-time, in-service course run by a local college of education, which includes special instruction in the roles of special education teachers. But unless adequate measures are taken to check out the backgrounds of the people placed in charge of the territory's children, it is possible that some teachers will continue to deviate from that role, and the most vulnerable sector of society will continue to be put at the greatest risk.