Nine of the teenagers who brought charges were pupils at Lagan College in Belfast and two were Army cadets.
The judge said there were aggravating factors in this case, among them abuse of trust and authority and the vulnerability of the victims, which Beckett knew about.
He said there were elements of grooming in Beckett's offending and that it continued after he was made aware of some of the allegations.
Judge Gilpin said "there was more to what you did than just the physical acts" and told Beckett he acted "for sexual purposes".
The court heard that Beckett continued to deny the offences and showed limited empathy for the victims in the case.
For the first time, many of the victims were in court supported by family.
They had all watched proceedings and given evidence from a remote evidence facility throughout the trial.
The judge also issued Beckett with a Sexual Offence Protection Order (SOPO) and gave restraining orders to all of the victims for five years.
He will be on the sex offenders register for an indefinite period.
In his trial, Beckett was found not guilty of rape, which was alleged to have taken place sometime between 2013 and 2015 while he was an Army cadet leader.
He was also found not guilty of sexual communication with a child and 10 further counts of sexual assault.
