I assume there was an element of 'plead guilty, save the cost of a lengthy trial and avoid putting the victims through it and the sentence will be reduced'. Was his age also taken into account I wonder?
And what exactly has he been convicted of? From the little I have read about it, the actual offences appear to be things like kissing, touching a breast and putting a hand up a skirt.
Perhaps 15 months doesn't seem so unreasonable when you compare those offences with serious sexual assault or rape.