The laws around consent have an area in them between 16 and 18 where (well between younger than 16 and 18 really) where there is allowance for prosecution due to power imbalances etc.
This is to recognise that while it is in no-ones interests to prosecute 2 15yo having consensual sex, while under 18 you are still very young, so leaves the door open for prosecution where there is power differential etc.
It is not as simple as "it is always legal for a man to have sex with a 16 yo girl" it is more complicated than that, because our society via our law says well actually there are times when it is not OK.
One of the times which is specified in law as not being OK is in a student teacher relationship:
"In relation to many other offences there is no requirement to prove an absence of consent. Only the act itself and the age of the victim or other criteria need to be proved. They include:
rape of a child under 13
assault by penetration of a child under 13
sexual assault of a child under 13 and
inciting or causing a person to engage in sexual activity with a child under 13
child sexual offences involving children under 16
children under 18 having sexual relations with persons in a position of trust
children under 18 involved with family members over 18
persons with a mental disorder impeding choice
persons with a mental disorder who are induced threatened or deceived
persons with a mental disorder who have sexual relations with care workers"
As you can see by the other offences it is grouped with, it is considered to be an extremely serious offence.
If you would like the law changed then the thing to do is raise it with your MP.