I'm going to reiterate this, as un-PC as it sounds: A WOMAN CAN NOT SEXUALLY ASSAULT A TEENAGE BOY. They want it, they fantasize about it, they high-five their friends, their dad is giving fist bumps...we're ruining two people's lives over something that's inappropriate but most likely harmless.
It's much tougher than that. Certainly cannot punish the
victim kid, no matter the gender, but the ethical issue here is that it is seen as an abuse of authority. Neither may see it that way personally, and you can weasel around in some way to say in some situations it is not that at all; but at the end of the day, it can complicate the teaching environment. And it becomes that much more of an issue if those kids don't know how to keep their mouth shut and, as you said, telling their friends, dads, etc. Now a not-insignificant portion of the school community knows, and if no one gets busted, that can really screw with the overall teaching environment and not just for that one class but for the entire school.
This is an issue that ethically you cannot just turn a blind eye to.
And for generally high school aged kids, it gets really sticky because everyone has a different idea of what the age of consent aught to be and may be fully willing to ignore the legal age of consent. Other times it's just pure raw hormones and the teacher goes against better judgment, because when any prime-aged adult gets revved up that's a battle against instinct. But that's the thing, we humans try to hold ourselves above basal instincts, because if you let that logic fly out the window it's a very slippery slope. We have tried to make it simple, cut and dry laws, this is the age, bam, no gray areas.
And frankly, why should we have different rules based on gender? The age-based limits are strictly based on adults over kids, general power/strength/mentality, and possible positions of authority and abuse of said authority. For the male teacher scenario, unless it's a full-on actual rape, there are plenty of times where it's a generally attractive male teacher and the teen girl is ready and willing and perhaps even instigated. Society is ready to flock to save the precious flowers but hell screw the teen boys they know what their getting into amirite?
I think we've reached a decent middle-ground over the centuries. Frankly we call young adults "adults" too early according to more recent scientific understanding of the brain, where the decision making center of the brain only starts to reach full maturity, on average, in the mid 20s. A fair reason why you often can't rent a car at 21 yet you can legally do everything else any other adult can.
edit:
Now, personally, I don't mind some of these gray areas, because yes, every situation is totally unique. Even a 30 year adult with a 17 year old, in *some* situations I expect there are zero issues and it's fully mutual. And again, really that's basal instincts and while yes we hold society to a high standard there's only so much we can to deprogram ourselves, as an adult in their prime (generally 35-40 and younger) and a sexually mature late-teen may very well find themselves lusting after one another. We have a problem as a species in that we reach sexual maturity way before we reach neurological/mental maturity. This makes it way too damn complicated for us, which is why we've debated for eons what is ethically/morally right, and frankly this will never be a truly settled matter.
I sure as hell know that I would have tried really hard to keep it a secret if I got to bang one of the hotter younger teachers I've come across, either as an actual student of said teacher or at the same school. I'd have been thinking about the repercussions for said teacher. But I cannot swear that I wouldn't have bragged to at least a handful of my closest friends hoping they'd keep the secret. But knowing them specifically and kids in general, they would've failed me. If the student really does the instigating, it's super hard to blame adults in their prime, of either gender. It's just really hard to have the capacity as a society to judge every single individual instance of this and know when there's been a grave misdeed and abuse of power as opposed to yeah just two hot bodies in heat (or the male equivalent, driven by second brain syndrome).