A former Bemidji assistant school principal has been given a 25-year sentence for posing on social media as a young teenage boy and directing dozens of girls and boys as young as 12 to send him sexually explicit photos of themselves.

Brandon Mark Bjerknes, 35, was sentenced Tuesday in federal court in St. Paul after pleading guilty in September to child pornography and coercion and enticement of a minor. Along with the 25-year term, Bjerknes will be on supervised release for life and must register with authorities as a predatory offender.

Bjerknes “repeatedly victimized at least 55 vulnerable young girls and boys over the course of almost three years,” Assistant U.S. Attorney Angela Munoz-Kaphing said in a statement following sentencing. “[The] sentence ensures that this defendant is held accountable for his reprehensible crimes and our community, including the brave victims who shared their stories, will be safe from his predatory actions.”

Bjerknes posed as a 13- to 15-year-old from Duluth named “Brett Larson” on Facebook and on Snapchat to contact middle school and high school girls and boys. He engaged in sexually explicit conversations and directed them to send him sexually explicit photographs. He also sent the students photos of his genitals and videos of adults having sex. At times, he used his school-issued smartphone.

Bjerknes’ tenure with the school district began in 2005, and he became assistant principal at the middle school in 2014. He resigned in April 2017 and was arrested in May.

Munoz-Kaphing said it was “a vigilant parent who initially reported [the] disturbing social media messages.”

Ahead of sentencing, defense attorney Peter Wold asked for a sentence of 15 years, arguing in a court filing that Bjerknes is unlikely to commit a similar offense.

“His crime has brought great shame on him and his family,” the filing continued. “Mr. Bjerknes does not want his family to be put through the public humiliation they have experienced ever again.”

The defense also pointed out that Bjerknes had no physical contact with his victims and “immediately accepted responsibility for his actions when he was arrested.”

In seeking a 30-year sentence, the prosecution’s presentence filing said Bjerknes “persuaded his victims to share their most private information. For example, multiple victims told him they practice self-harm, cutting behavior; two victims reported they were transgender; and other victims shared information about stressful situations at home.

“Then Bjerknes leveraged the information he learned from and about the victims to sexually exploit them for his own pleasure.”