When I was at school, the PE teacher had a "relationship" with a fifth year (she was 16) and, as far as I know, they are still married.
Not any longer, section 16 of the Sexual Offences Act 2003 created the offence of Abuse of position of trust: sexual activity with a child. Without going through it word by word a teacher having any sexual activity with a pupil under 18 who attends the place of education at which he teaches faces up to 5 years of "re-education" at one of Her Majesty's penal institutions.