When I first found this site I remember coming across a managa where the description was of a
single sex school which went co-ed, but only one student of the opposite sex joined.

I think it was a lone female student in a previously all boys school, but it might be a lone male student in a previously all female school.

Unfortunately I cannot remember the title, nor have I been able to find it again.

Ah! Itoshi no Banchousama?

Thanks for those two, will check them out.

As I find is usually the case after weeks of searching I go and find it a couple of hours after asking for help.

Ookami Nanka Kowakunai!?

and for good measure I found this one as well.

Ichiban!!!

There are quite a few like this. I think the school in Starry Sky only has the one girl (the protagonist), which makes sense given it's a reverse harem otome game.

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