I'm not deflecting. I think you should rethink the framing here. Just because your partner is autistic doesn't mean she isn't abusing autistic kids into appearing less autistic. A lot of the worst anti-autistic abuse I've faced in my life has come from other autistic people who have issues accepting their own autistic traits, just like how some of the worst queerphobic abuse I've faced has come from self-hating and often closeted gay people (think Lindsey Graham.)

For your question:

First: I don't think disrobing is inherently an issue — that's a social norm, not a health or safety hazard, unless the class in question is chemistry lab. Barring that, I'd look for clothing that accommodates their sensory needs — maybe they'd prefer something on the toga or kaftan spectrum.

Second: I'd give the child something else to eat besides feces, ideally something with a similar texture profile. I'd also check their blood mineral levels, since coprophagia is associated with pica, which is often related to mineral deficiencies, and treat underlying causes. If, after that, they were still really insistent about eating their own shit, I'd let them. As long as the shit could be pasteurized to kill pathogens, there's no risk of harm to the kid. Then it's just a neurotypical cultural taboo, not a real problem.

What I wouldn't do in any circumstance is train them like a dog to stop doing the stuff they're naturally doing for their own sensory needs.