A male teacher was caught 'in a passionate embrace' on a classroom floor with a 16-year-old pupil shortly after a parent-teacher meeting.
James Mullen, a 23-year-old music teacher at the £24,000-a-year Catholic boarding school, first seduced the schoolgirl on Valentine's Day, six months after he joined the school, a court heard.
He groomed the 'vulnerable' girl by sending her text messages and saying he loved her.
At St Albans Crown Court, Mullen, from Rotherham, was jailed for eight months on Friday after admitting three charges of sexual activity with a child while in a position of trust.
The relationship began earlier this year just a day after Mullen was spoken to by senior staff over concerns about his behaviour with pupils, such as watching DVDs and sharing pizzas on school property.
On Valentine's Day, February 14, the schoolgirl said they shared a 'passionate kiss' after having first developed a friendship.
They embraced on one more occasion before their relationship was uncovered when another teacher saw them in a 'passionate embrace' in the school's music room at 10pm on February 26.
She confronted Mullen after seeing the girl leaving the room in a 'dishevelled state'.
He broke down and was arrested by police later that evening.
The girl told police that Mullen had rubbed her chest area over her clothing while they embraced in the music room. She also told police they expressed their love for each other the same evening.
Text messages between them proved he knew the relationship was wrong, the prosecution said.
In one, the girl asked Mullen, 'Is this law breaking? How much trouble could we get into?', to which he replied, 'Unfortunately, with me you are classified as vulnerable because I am your teacher'.
The girl had also sent Mullen a text which said: "It feels so right – I am not giving that up."
It also emerged in court that Mullen had been sent on a child protection course when he joined the school which he passed with 100 per cent.
Mullen was placed on the sex offenders register for 10 years, banned from working with children and made the subject of a sexual offences prevention order.