"magistram" is also accepted? Why is that so?
Usually acc. form is used but an exception is when you use the verb "esse/sum".
E.g. Ego puella sum. ✅
Ego puellam sum. ❌
It's how Duolingo handles the 'misspellings'. From what I understand, the contributors themselves cannot set it up to mark for grammatical case, like mark magistram as incorrect in this case (they have to deal with the Duolingo team or something).