Translation:My chalk is lost at the school yard.
Di isn't simply "in", it marks a location. In English, this can variously be expressed as "In", "at", "into", "on", "on to", etc depending on the context and surrounding grammatical syntax
Just because the answer is given in an active voice it should not be marked wrong, i.e. "I lost my chalk in the school yard".
I think it's a bit of a grey area and depends on how easy it is to directly translate a given sentence in the given voice. The voice chosen to express something carries a LOT of nuance, so it should be maintained unless the translated sentence is just too awkward. Personally I think "My chalk was lost" reads ok. I find that a lot of sentences that sound awkward in isolation can sound perfectly fine with the right context.