You can't use "the law" as an infalliable guide to the morality of a given situation.
Forrest undoubtedly abused his position. I absolutely agree that teachers should not be having sex with their pupils - although I'd have to think more about university students and tutors I suppose.
However, those who are using the fact that it is illegal to form a moral judgement have a weak argument I think. The law has said all sorts of things about the age of consent - in 1275 it was 12, and was 10 by 1576 and was 13 in 1863 but raised to 16 in 1885. In Canada it was 14 until 2008, when it became 16. In America it depends on which state you live in. In Mexico it is 12.
So 'the law' is a flawed guide to what is and isn't moral.
I think his behaviour was immoral because of his authority over her, not because of her age per se, although that is a factor in his authority. It would also have been immoral if she were 25 and he was her parole officer, for example, or if she was 50 with a mental age of 12. Her age is a factor, but it's not the key factor - he was wrong because of their relative positions, not simply because of her age. I think the law needs to protect younger people - and should (and does) protect students from their teachers - but the idea that having sex with a physically mature 15 year old who is a consenting partner makes you evil is a difficult one to codify. Would it be immoral for a 16 year old boy to have had sex with her the day before her 16th birthday? What about a 17 year old? Where is the line? And why? (and please don't forget the bits about a teacher - pupil relationship always being wrong before you reply)