Many people who are commenting are just missing the point.  This case as nothing to do with the age gap. The real point is that Forrest is a teacher and  has misused his position of power over a pupil.  Whether they are in love or not  is irrelevant.  At the age of 15 Megan is not deemed, in law to be capable of deciding to enter a relationship with her teacher. In this instance she would need to be over 18 years of age and not the usual 16 years before she was legally able to consent to the relationship. 
Rightly or wrongly, this is for the protection of both pupils and teachers.  If the love is strong, it should last until Megan is over 18, if not, was it love in the first place and it may have been just infatuation.