Most people on her presume that it would have been some kind of one off. There is no evidence of this, he clearly found young girls attractive and there is nothing to suggest that he would have stopped at this 15 year old girl. Who knows how many of his 13/14 year olds that looked a bit older than they were he found attractive and would have fallen in love with?
He groomed an underage girl, it is not difficult to avoid a situation where you will fall in love with a pupil. I very much doubt it just happened one day. They were right to severely punish him and send out a message to any others.
You may well be right but is not think there is any evidence to suggest that Forrest would go on to commit similar crimes with other teenage girls.