"Copiii au displăcut-o pe profesoara lor."
Translation:The children disliked their teacher.
Didn't realize that plăcea/displăcea could even be used with a direct object. Wouldn't it be more common to express this with the dative case? That said, I'm not sure what that would event look like. Something like 'Profesoara le-a displăcut copiilor.' or 'Profesoara copiilor le-a displăcut.'?
A Romanian speaker would understand what you mean by using "a displăcea" as a transitive verb, but this is not standard nor common usage. I think Duolingo is pushing the boundaries of the language here. (Or just plain wrong.)
The correct way to say the above sentence is either of the following:
- Copiii nu au plăcut-o pe profesoara lor. = The children did not like their teacher.
- Copiilor le-a displăcut profesoara lor. = The children disliked their teacher.
PS: Your versions are also okay, but they could mean slightly different things depending on context and emphasis. For now, you should stick to the ones I wrote above.