Translation:My younger sister goes to school at 11 o'clock.
Correct me if I am wrong but I think the accurate translation is "starts school/class/studying" To go to school is 去学校
Is it "goes to school" (as in, leaves for) or "gets to school" (as in, arrives at)? I always thought "上 + place" means to arrive at.
I might be wrong, but I think that's because there is nothing indicating whether its am or pm. And while it's unlikely she goes to school at 11pm (well 11am isn't that likely either) you can't add am if it's supposed to be an exact translation.
I typed "my little sister goes to school at 11:00 o'clock" and my translation was not accepted.
Duo usually likes you to include the "younger", "older" etc when talking about brothers and sisters, although sometimes they allow it without them. Maybe that is your problem here.