A teenager was forced to perform sex acts on an RAF cadet sergeant in return for a lift in his car.
Hayden Carnall, 19, lied to his vulnerable 15-year-old victim he would take to her Blackpool home - but forced her to repeat the act again.
He was already on bail on suspicion of sexual offences against another youngster.
The former St George’s pupil, now of Lulworth Road, Southport, was jailed for six years at Preston’s Sessions House Court.
His other victim was just 13 and her family had witnessed her change from a top pupil to an “unrecognisable” girl who self-harmed, was troubled at school and needed counselling.
He admitted meeting a 15-year-old girl following grooming and two counts of sexual activity with her, and sexual activity with a 13-year-old.
Two counts of sexual activity with another girl were ordered to lie on file.
The offences happened when Carnell was aged between 17 and 19.
Sentencing, Judge Graham Knowles QC said: “You said age doesn’t matter. It does.
“These were schoolgirls. You knew what power, what attraction what sophistication you appeared to have. It is not a case of teenage sexuality gone wrong or a youth with a girlfriend a little younger - it is a case of predatory sexual abuse.
“Everyone knows that 17 year old young men or boys should not have sex with 13 years olds. It is morally wrong.
“It was a nasty piece of offending.
“You knew you were committing sexual crimes so any immaturity on your part counts for little. These were impulsive sexual crimes, these were planned and deliberate offences. “You may not have understood how much harm you would do to these girls.”
Carnall will be entitled to be released on licence after serving half of his sentence, and will be subject to notification and barring requirements indefinitely.
An indefinite sexual harm prevention order restricts his internet use unless his history can be made available for inspection, and having unsupervised contact with any girls under 16.