He gets longer because he was in a position of trust. A shool teacher for example would have been handed a similar length sentence.
Don't get me wrong, he deserves prison but it does seem a stern sentence, your theory doesn't really follow. My very own art teacher only got 9 months for full sex with a 14 year old pupil.
http://www.independent.co.uk/news/uk/art-teacher-gets-jail-term-for-unlawful-sex-with-pupil-14-1392870.html