A Calgary teacher, described as a "serial sexual predator," has been sentenced to 16 years in prison for victimizing 26 children.

Christian Sarile, 29, pleaded guilty to 17 charges involving the sexual abuse of 25 underage girls and one transgender boy over an eight-year period.

"He is clearly an admitted serial sexual predator who exploited and assaulted vulnerable children for his own sexual gratification and amusement," said Court of Queen's Bench Justice Earl Wilson as he began his scathing sentencing decision Friday morning.

Wilson described Sarile as "evil, manipulative" and his actions as "relentless and callous."

Defence lawyer Yoav Niv proposed a 12- to 14-year sentence for his client, while prosecutor Martha O'Connor argued Sarile should spend 18 years in prison.

Grade 6 teacher

Sarile's victims ranged in age from 12 to 17 years old. Often he met them through social media, sometimes using class lists from the schools where he taught to pick his targets.

He paid some of the children for oral sex. More often, he had his victims send him nude photos and if they tried to stop, he would blackmail them until they complied with his relentless demands.

When police first arrested Sarile in May 2017, he was a Grade 6 and music teacher.

Sarile admitted to paying his victims in money, drugs, alcohol and vaporizers in exchange for sex and nude photos.

His convictions include charges of sexual assault, paying minors for sexual services, sexual extortion, luring and making child pornography.

Sarile blindfolded children, took video

On one occasion, Sarile paid three girls, aged 12, 13 and 14, to give him oral sex in the back of his van, but he was worried they would be able to identify him as a teacher so he blindfolded them.

Police found a video of the encounter, showing three blindfolded children taking turns giving Sarile oral sex.

In several cases, Sarile would deliberately target children who were particularly vulnerable — girls who had troubles at home, mental health issues of substance abuse problems.

While pretending to be someone else, Sarile would contact his victims and threaten them if they did not perform sexual acts. Fearing he would distribute their nude photos, some of the girls gave in and had sexual contact with Sarile.

The judge noted that the victim impact statements "detailed the absolute hell on earth that this man created for these children."

Sarile was first arrested on May 2, 2017, and immediately admitted to his crimes. After a press release was sent out by Calgary police, dozens of new complainants came forward.

After he was released on bail, Sarile sexually assaulted another 14-year-old girl and was rearrested on Dec. 7, 2017. He's been in custody ever since.

'Something wasn't right'

The two police officers at the heart of the investigation said it was one of the largest ever undertaken by CPS's child abuse unit.

Sgt. Mark Weir began as lead investigator alongside Det. Jason Smith, who took over when his partner changed units.

Initially, they had no idea that based on a complaint by one girl's grandfather, 25 more young victims would come forward to tell their stories.

"We had that feeling that something wasn't right here," said Weir.

After police released the names used by Sarile online, a flood of victims called in.

"That's when this thing really blew open on us."

Sgt. Mark Weir, left, and Det. Jason Smith were in charge of the investigation into Christian Sarile, which police dubbed Operation Choir. (Meghan Grant/CBC)

With the sentence now handed down, Smith calls the conclusion of the case "a big relief."

"It was very draining on a lot of kids, a lot of investigators, even the Crown. A lot of families are very very relieved that their kids can close the book."

The two officers say there is one big lesson to be learned here and it's about how parents monitor their kids' use of social media.

"If your children are on social media, on any kind of internet platform, we need to be aware of what they're doing, what they're looking at," said Smith. "It's not the stranger who's doing most of the offences against children. It is often someone they know. So you need to know what your children are doing online to protect them."

Weir has since changed units and now works with kids in an effort to help prevent them from becoming victims.

But he says that in his 17 years as a police officer, the Sarile investigation is "one I'll remember for the rest of my life."