Derek Siddon, 32, who taught music at the school, pleaded not guilty in the ACT Supreme Court yesterday to 10 counts involving sex with a person under 16.

He had originally been charged with three counts of sexual intercourse and five counts of attempted sexual intercourse with the student, and was committed to stand trial after a committal hearing in June last year.

The charges were upgraded in a pre-trial arraignment before Justice Richard Refshauge yesterday.

The offences are alleged to have occurred between June and August 2005 when the student was 15, but details later emerged that the relationship continued for another two years, after she had reached the age of consent.

The woman, who cannot be named, said Siddon had first propositioned her in April or May of 2005, when she was in Year 10, telling her he had feelings for her, supplying her with condoms and telling her he wanted to have sex.

After she agreed to sleep with him, the two attempted sex a number of times, and had intercourse at Siddon's friend's house, as well as once in an empty classroom at the school.

During her evidence at the committal hearing, the 18-year-old said a teacher had questioned her about the relationship in 2005 and that she lied.

She said she had been worried about Siddon's reputation and about being expelled, and did not want her parents to find out.

She maintained throughout her evidence that Siddon, who was married with two children at the time, had been aware she was under 16 at the time of the alleged offences, because she had written her birthday on his office calendar.

The trial is scheduled to start in August.

Meanwhile, another former Canberra teacher, 43-year-old Tania Tominac, is due to stand trial next month, charged with having sex with a 14-year-old male student while she was a teacher at St Francis Xavier College in 2005.