My student completed a test on Duolingo and received 890xp. Is this normal?
That is possible, if a student already has a good grasp of a part of the target language
Yep, if he or she took the placement test and already new quite a bit of the language. He or she is probably set pretty far into the tree, perhaps near half? Or a little less? Anyway, you should not worry about it. It's perfectly normal and common.