It's mostly a matter of professionalism (specifically, professional appearance) and reputation.
While activities outside your profession shouldn't affect it, they can come back to haunt you and damage your reputation. Depending on how serious these allegations are and if they're found to be true, your professional liscence could potentially be revoked as a result. This seems unlikely to result in the latter, particularly if being in an adult film was done before going into teaching. Currently doing so? ... Eh, dicey territory. Still, I don't see the liscensure being a problem here (not sure if teachers require a liscence in the area, but I'm assuming they do).
The matter of reputation is a bit more prevalent, as even false rumours can easily ruin that for you; regaining a lost reputation is extremely difficult, and impossible in some cases. This could result in the teacher losing her job so that the school (and/or school district) can save face; she could easily seek employment elsewhere, but any employer would be weary of hiring her for the exact same reason. It's not that she would be a bad teacher or even a bad influence (arguably quite the opposite, she could be very much against others doing what she did due to her past experiences; and unlike most, would know exactly why), it's just that employer would be worried about their own reputation and appearances.
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I personally don't see an issue here, though I would prefer to see the teacher in this case try to dissociate herself (as a teacher) from her past in the adult film industry. The reasons for this are for the sake of her own reputation (so that parents don't jump the gun and the current situation hopefully doesn't happen) and so that the kids (namely, the boys) don't hound her constantly about it (should be easy to see why) and she can't teach them effectively as a result.
It all comes back to the fact that morons tend to be loud and obnoxious, preferring to hop on the bandwagon instead of thinking things through. Speaking of which, as someone about to enter profession myself (and fortunately not subject to such circumstances), I forsee my own biggest challenge to be not calling someone an idiot when there's every reason to do so; professionalism can be quite the annoyance at times, but it is there for a reason. Regrettably, those complaining are subject to the same code of conduct.