This is also correct. It depends on whether "all girls" are subject or object in the sentence. Since we don't know in this case, it could be any of the two.
Alla flickor utom jag springer - All girls except me are running. Han ser alla flickor utom mig - He sees all girls except me.
In English it's the same as Swedish, and there's another thread where it's discussed. Used as a subject "All the girls except I are going to the movie" vs object "Annika's mom took all the girls except me to the movie." All that said, it didn't accept "All the girls except I" ... so I've reported it.
I'm not talking about "alla" in the entry above.
Alla - means "all" when talking about several things or people.
Allt - means "all" when talking about t-gender words as a mass.
All - means "all" when talking abot n-gender words as a mass.
Hon har alla böckerna - She has all of the books
Hon har allt vatten - She has all of the water
Hon har all juice - She has all of the juice
It is important to add here that alla and allt have additional meanings.
Alla - Everybody
Allt - Everything (Short for allting)
Alla kom på festen - Everybody came to the party
Allt är borta - Everything is gone
Well, you have to draw the line somewhere. I think that when teaching a language it's good not to be too permissive. A reasonable rule is that if a literal translation would result in an undiomatic text, then some paraphrasing should be allowed.
But that is not the case here. Here there is an exact English translation for the Swedish, and it is 'all girls'. Furthermore, there is also an exact Swedish translation for 'every girl', and it is (I believe) 'varje flicka'.
So I think that in this case, Duo is right to insist on the literal translation.