An award-winning jazz musician who worked at a prestigious Victorian high school has admitted sending 150 sexually inappropriate messages to a schoolgirl in just one day.

Former Nossal High School director of music James Mustafa sent the flurry of texts to a 16-year-old VCE music student on May 30 last year.

James Mustafa fronts court after pleading guilty to being sexually inappropriate with a child in his care. Joe Armao

He asked the Year 11 student about her sexuality and sexual practices and encouraged her to sext back.

The case raises serious questions about an apparent loophole in Victoria’s teaching regulations.

The now 26-year-old was not required to register with the teaching watchdog, the Victorian Institute of Teaching, because he was employed by the school as an education support worker.

While music teachers must be registered and vetted by the regulator, there is no such requirement for education support workers.

Mustafa was awarded the prestigious Young Australian Jazz Artist of the Year award in 2015 and has shared the stage with James Morrison and collaborated with Kate Ceberano.

At the Ringwood Magistrates Court on Wednesday he pleaded guilty to one count of encouraging a child aged 16 under his care to engage in, or be involved in, sexual activity.

The horn player, pianist, composer and former member of brass band Horns of Leroy was arrested in July at his Croydon home.

He admitted to police that he was encouraging the girl to participate in sexual activities and was aroused by the text messages.

James Mustafa's promising career is in tatters. Supplied

Magistrate Michelle Mykytowycz said the victim’s personal and professional life had been significantly impacted by Mustafa’s behaviour.

“Her professional pathway has been completely stymied and she is having to reconsider and rethink that,” she said.

Outside the court, the victim said the text messages had made her feel incredibly uncomfortable and it had been difficult to return to school.

"I felt like I was in a very vulnerable position," she said.

"No one should have to go through that."

She said while she was initially scared to report the incident to police, she was glad she took action.

"At the end of the day, it gives you so much strength, taking back your story.

"It does not define you at all and people need to be held accountable for their actions."

Ms Mykytowycz told the court that Mustafa has abused his position of authority and knew his actions were legally and morally wrong.

In a statement read to court by his lawyer, Mustafa said he knew he had "stepped over the line" and done the wrong thing.

“She was my student and I had a duty of care ... I just didn’t care at the time,” he said.

Mustafa’s lawyer said his client was profoundly remorseful and had excellent prospects of rehabilitation.

He described Mustafa as one of the state’s “most talented, multi-instrumental musicians”.

But he said his career had come to “an abrupt and crashing halt” since he’d been charged.

He said his work as a composer had dried up, contracts had been cancelled and he’d been disowned by friends. The court heard that Mustafa had received death threats and he was “close to unemployed”.

On a good week, the court heard, he receives $600 playing the piano at funerals.

According to Mustafa's recently deleted website he also worked at Gippsland Grammar and his former school, Wesley College.

Mustafa will be sentenced on Friday morning, with Magistrate Mykytowycz considering either a prison sentence or community corrections order.