Protecting children

In UK news recently there has been a lot of fuss about a teacher running away to France with a 15 year old 'student' (or to translate to English, "15 year old student" = schoolchild).

There are several dichotomies here but I will focus on two.

1. Students are students, but 15 year olds are school children and they should be protected to a greater degree.  I think the normal agreed age for adultdhood is 18 and that the teacher behaved inappropriately.   However  . . .
2. In other cases, elderly muslim men marry 7 year old 'brides' and yet there are no prosecutions.

Is justice served well?