The crux of the matter is that teachers are in a position of influence whereby they can bring pressure to bear on their charges thus whilst the sexual activity may well be between two people over the "age of consent" the consent from the pupil may well be given only after inappropriate influence has been exerted by the teacher and would not therefore be informed consent but rather coerced consent.
I heard this being debated on the radio earlier and the union idiot was back peddling furiously and saying that any sex between a teacher and a pupil, no matter what the age, should still be a matter over which the teacher should lose their job and not be allowed to teach again EVER but they should not be placed on the sex offenders register.