Translation:My wallet was lost when I went to school.
The translation here should include "My wallet went missing when I went to school." It is a passive construction and should not be translated as "I lost my wallet etc" - that would be "Saya menghilangkan dompet saya ketika saya berangkat sekolah" - even if in English an active construction would be more natural.
i agree with the comments here. there has been a curious tendency to phrase things more passively such as 'my wallet is lost' rather than 'i lost my wallet'. this clearly makes translating into natural english a little awkward at times.
i wonder if that reflects a preferential way of speaking in indonesian? or if it is simply the result of the limited grammar we have encountered at this stage in the course that forces this form of sentence structure?
I would probably translate this as "My wallet went missing when I left for school". Although, I asked my husband (a native Indonesian) exactly what situation this sentence is describing and he said the wallet was lost while on the way to school, not while at school or not in the home when leaving