I think a couple of things prevents it from being overly common.

There's the tendency of people to favour the first accuser. If someone comes running up to a stranger and says that the person chasing them is trying to assault them, then the person chasing them says it's not true and that they stole their wallet, people tend to favour the first accuser and assume the second one is just trying to cover their tracks. Think about it in a work place HR setting. If someone reports you to HR for bullying them, HR tends to take the first story even if you come in, say that's not true, they were bully you and you'd been silently putting up with it for moths.

So, when a kid and their parents come out and say the teacher has been grooming the child, it already feels too late to suddenly burst into tears and say you were being raped.

There's also the believability of the story. Are there text messages and emails about them agreeing to meet? Had he been "Raping" her for 4 months now, at her house, after school each Friday? Why hasn't she told anyone? How is she still teaching a class and treating him so well after this? Why are other students saying they always seemed very close and friendly? This isn't her waking up after a bender in a stranger's bed, knowing her boyfriend will be furious, so deciding it was rape and running to him crying - this is a long term relationship that half the class secretly suspected. They simply might not feel they can get away with it and know it will land them deeper if they try.

And lastly, and most messed up maybe, is that I think they have spent a lot of mental energy justifying what they are doing already, and this suddenly backflip on it just won't bridge that mental gap - Bear with me on this one:

A male teacher might have sex with a student because she's young, beautiful, sweet and he just gives into the temptation when she shows interest. It's not love, he just didn't have the self control and willpower we expect from a common adult to not just plough anything, regardless of age. I suspect a lot of them are lonely, love starved, in a world where men are expected to meet a certain standard and they simply don't, so this opportunity just boggles their mind and they dive in... Women, on the other hand, tend to "Fall in love", as a rule, when it comes to relationships and sex. That's not to say women never have casual sex with someone they just find attractive, but in general, when it comes to sex, women typically talk about connection and emotion and love more than they talk about just not being able to resist that tight butt and those strong arms. We know that isn't 100% genuine - women still have sex for lust and then pretend it was love - but as often as not, they are jumping through those hoops for themselves as much as anyone else.

I think, therefore, that a lot of these female teachers who have ended up with these students have done a lot of internal, mental gymnastics to convince themselves that this is true love. "Why would I have sex with a student, who is underaged and in my class, unless he was very special and this was true love? Nobody else understands, but this isn't just sex. We're meant to be together. He understands me like no man ever has." etc etc etc... For some of these female teachers, I imagine, they have attempted to convince themselves of all this so very hard that the idea of now just doing a 180 and claiming it was rape to save themselves is as damaging for their own mind as the pedophilia charges. They need this to be true love that's just misunderstood by the world and is tragically romantic in order to avoid facing the reality - they groomed a child.