By and large for something to be a crime, there has to be a victim, and it's the degree of harm that, to us, controls how harsh we view it, along with the perpetrator's reasonably being able to expect such damage. Damage for things of this type is primarily to the mind, not the body. Can't have much of a crime without a victim, and you can't have a serious crime with out correspondingly increased victimization. That victim can be an individual or society, but relies around the idea that the perpetrator can reasonably be expected know something is that harmful, and that means the victim must actually be victimized. A 17 year old boy is just not, in most such cases, reasonably viewable as a victim nor his teacher as a great criminal. We roll our eyes at the boy's 'success' and regard the woman as a fool worthy of contempt who needs to be fired and maybe punished further. But that's just the general case, the default, most of us on hearing greater details will evaluate it case-by-case. Culture sets morals, and morals tend to determine laws, by and large we just don't see cases like this and say "She should have known she'd seriously traumatize the boy, and look at how damaged he is, bragging to his friends and all."