Giving evidence in person at the hearing the girl, known as pupil A, confirmed her police statement which was read out by presenting officer Damian Phillips.
The girl described how she had been bending over a printer in an end-of-year science class in July 2004 when Mr Towers approached from behind.
She said: “Then a felt a firm slap on my bottom. I heard him say “you naughty girl”.
“I felt really upset and just wanted to get out of the classroom.
The teenager, who now works as a receptionist, broke down in tears at the beginning of her evidence which described how she had felt “embarrassed and humiliated” in front of her school friends.