A TEACHER has been arrested for allegedly forging documents claiming a headmaster who sacked him had contracted AIDS. The former English teacher at a Tuen Mun school, in his late 20s, is alleged to have forged three documents claiming his former employer Mr Ying Yu-hing, also a Tuen Mun district board member, had contracted the disease. The documents, one of them a letter to Director of Education Mr Dominic Wong Shing-wah with Mr Ying's forged signature, have apparently been sent to schools in Yuen Long and Tuen Mun. The letter claimed Mr Ying had been sacked by the school board after he was tested positive for the AIDS virus, portraying Mr Ying as a victim appealing for the director's help. A bogus Prince of Wales Hospital document ''confirming'' the condition of Mr Ying and a blood test report were enclosed. Last week, Mr Ying said the Hospital Authority had written to him denying the document came from the hospital. The suspect, a UK graduate, was arrested at Kai Tak airport on his return from Australia. The police source said the teacher and Mr Ying had always had a bad working relationship. The teacher was sacked more than a year ago. A 90-page report from the school to the Education Department said the teacher was fired after a grave breach of duty. Mr Ying confirmed the teacher had been the subject of another police investigation when a student and his parents reported ''a conflict'' to police last year. ''But before he appeared in court he was sacked. The school board made the decision for professional and ethical reasons, independent of the court case.'' Mr Ying said he regretted this had happened, but denied the quality of teachers had dropped.