Is wanting a co-ed school valid grounds for appeal?

[Mummmypirate]

Or vice versa? Can you effectively be forced into single-sex or co-ed school by allocation? Just wondering if anyone has successfully won an appeal on basis of this objection?

No. That is about what you want. Your appeal must be about why your child will be disadvantaged if he/she is not admitted to this school. You are appealing for this school, not against the allocated school. Unless you can show that your child will be disadvantaged if they go into a single sex school (unlikely and would need expert evidence to support) you would not win on these grounds.

Not really, because - as prh says - that's more about the parent's wishes than the child's needs. There may be instances where a chid has a need for a single sex or mixed school and can provide supporting evidence of that from professionals, but I can't readily think what those instances would be so imagine they'd be very rare.