In the United States if one would say half past nine, (although common until the mid-twentieth century), listeners would assume that you had been reared on an isolated mountain with little interaction with civilized individuals. Now that may be a bit of a stretch, but in today's digital world the time would be presented as 9:30. Dúo is still not accepting it.
This question is a mess. The only thing that's accepted is the exact statement: Class started at half past nine I had the following marked incorrect: Class started half past nine, Class started at nine thirty, Class started nine thirty, Class started 9:30, Class started at 9:30