It should be possible to achieve separate teaching without exclusion, shouldn't it?
How does that meet the very clear need of the victim to feel safe? How can that child or young person feel safe, knowing that the perpetrator is in the same building, when they must attend that building every single day of the school year? Where, and how, are her own needs being considered, far less met, in this analysis?
If there is any even slight harassment, even borderline comments, this is the point at which prevention should come into place. And if one boy has harassed or assaulted someone, proper training on toxic masculinity should be happening immediately. Excluding one boy will be no deterrent at all to others - a school that has excluded a boy for sexual assault and "read the riot act" to the others is still an unsafe school for girls.
This is the same argument used as to why women don't need single sex provision. The problem is toxic masculinity, so let's address that in a fantasy world where everyone agrees that the slightest comment or joke should be instantly leapt upon (and who decides where that boundary lies, anyway - or who is telling the truth, in reporting such comments?)... and meanwhile, women can suck up the risk while we all await utopia.
The same misplaced belief that you can train someone out of being predatory led to such sessions in prisons. The ones that were found to increase offending behaviour, upon release. By the time a teenager is sexually assaulting peers, it's a tad late to try 'proper training on toxic masculinity'. They need a level of support and intervention that is not available in the local secondary school, and nor can it be.
A new setting and a new start is required for any perpetrator, with a support level calibrated at trying to prevent reoffending. And the mental health needs of the victim should come first. That means providing a space that feels one hell of a lot safer than any can, with the presence of the perpetrator.
1:1 provision in a mainstream, and otherwise social isolation, is not inclusion, anyway. It's not going to do a damn thing to support or change their behaviour. Kids who do this sort of thing to a level where exclusion is deemed proportionate need a therapeutic environment and proper oversight, not just leaving in a mainstream setting to prove an ideological point.