Billie-Jo trial hears of temper.
Sion Jenkins attacked Billie-Jo with an iron bar as she painted doors at the family home, the court heard.
He hit her at least five times and it was 'merciful' she was unconscious, prosecuting barrister Nicholas Hilliard told the jury.
The reason Jenkins attacked his foster daughter was because she had annoyed him in some way, said Mr Hilliard.
The youngster may have messed up the painting or turned up the music she was listening to, the court heard.
Mr Hilliard was opening the case against Jenkins, 47, of Aberystwyth, who denies murdering Billie-Jo on February 15, 1997.
Before the case opened, trial judge Mrs Justice Rafferty told the jury of six men and six women there had been previous legal proceedings.
'This is a completely fresh start,' she said and warned them to try the case solely on the evidence brought before them in this trial - 'not on anything else'.
Mr Hilliard said Jenkins had once been seen by a friend on holiday kicking Billie-Jo after she twisted her ankle. Jenkins accused Billie-Jo of attention-seeking.
'It is a particularly cruel thing to do, particularly to a child,' said Mr Hilliard.
If the allegation had got out, 'the consequences would be pretty drastic -Billie-Jo removed, his life over and his career in ruins', said Mr Hilliard. 'It must have been a rather tense time for the defendant. '
Jenkins had got the deputy head's job at the school in 1992, and applied for the head's job in 1996. But 'he had not been entirely truthful in his application form', said Mr Hilliard.
Jenkins had said he was educated at Gordonstoun, claimed he had 10 O'levels and three A levels - but this was not true, the jury was told. He had also been untruthful about his degree and teaching qualifications.
Jenkins had been told he had got the head's job subject to references but confirmation was taking a long time - causing him to be gloomy, the court heard.
Jenkins's ex-wife Lois would be a witness in his trial, said Mr Hilliard. She would give evidence by video link from abroad.
The couple have four children - now aged 20, 19, 17 and 15.
Billie-Jo went to live with the family when they lived in east London in 1992.
The trial continues.