Christopher Reen, 32, of Mount Drive, Bridlington, pleaded guilty at Hull Crown Court to six charges of sexual activity with a child.
Reen admitted the charges before a retrial could begin on Monday, as the former pupil waited to give evidence aga
inst him.
A jury had failed to reach a verdict in a trial last September, which saw married Reen admit one charge of sexual activity with a child while abusing a position of trust.
Reen had directly replaced a female teaching assistant at Headlands School who had appeared in court for an inappropriate relationship with a teenage boy.
He had been specifically warned that four Headlands teachers had been imprisoned or dealt with in court for teacher-pupil relationships in the past few years.
At the time of the affair, Reen had just married and his wife was pregnant with their first child.
His barrister, Anil Murray, told the court Reen admitted the affair,
claiming he was wreckless about whether he reasonably believed the girl was under 16.
Crown barrister Helen Hendry had earlier told the court Reen first met the girl when he took her for a class in child development.
"Twice in that class, he sent her text messages knowing that she was a pupil in the class," said Mrs Hendry.
"He says she deliberately misled him, saying she had been held back a year. The prosecution says that was a bare-faced lie."
Mrs Hendry said Reen contacted the girl via a text message and, as the affair progressed, the pair exchanged 800 messages, with Reen sending the girl explicit pictures and messages.
She said Reen and the girl had sex in his family home and after classes at Reen's out-of-school karate club.
Mrs Hendry said rumours of the affair soon spread to teachers and the girl's mother and the police were informed by the school.
But the girl was reluctant to prosecute until she realised she would have to give up her mobile phone.
Mrs Hendry said Reen mocked the school's efforts to remind staff about child protection.
"He appeared earnest, anxious and self-righteous, but you will recall her evidence, he laughed when he told her about the staff reminder," she said.
"Here was a school that was doing all it could to protect itself from further scandal that had dogged it for the last few years, and here he was laughing at their attempt to protect children."
Defence barrister Anil Murray admitted Reen's career as a classroom supervisor was now over.
Mr Murray told the court: "This is not a case where I can beat my chest and ask for justice for an innocent man.
"His behaviour in the summer of 2008 was totally reprehensible, shameful and criminal.
"He will leave court a broken man whose life is in tatters."
Mr Murray said Reen had let everybody down and should have shown more discipline in protecting the girl from himself.
"This relationship was never going to end well. And perhaps, most of all, he has let himself down.
"Perhaps he revelled in the excitement.
"Whatever he did he brought his own world crashing down.
"He will be loathed even more than he is now and he will be called a paedophile.
"He will have to watch the pain inflicted on those that he loved and he will know that it is all his own fault.
"And he will have to live with that all his life," added Mr Murray.
The case was adjourned for a pre-sentence report to be prepared.
Reen will be sentenced at a hearing next month.
Judge David Tremberg told Reen: "I make it absolutely plain to you, so you are under no illusions, you must, when you come back, expect a substantial sentence.
"It seems to me the sentencing guidelines leave me no option."
