I wrote "η ώρα είναι έντεκα" and was marked wrong. Is order strict in this case?
I wouldn't say so. "Η ώρα είναι έντεκα" seems just fine to me. It's been added. ^.^
"Είναι η ώρα έντεκα" wouldn't work?
It sounds a bit awkward, to my ears. A bit old fashioned too, maybe.
I wrote "ένδεκα" instead of "έντεκα". Is that wrong in this context?
No, it's fine. I've added it now; thanks!