"At that time I was at school."
Translation:В это время я была в школе.
The literal translation (which is what Duolingo is asking) is "at that time" and "then". The meaning is the same however.
No, because время is neuter. (I know this is from years ago, but other users could have the same doubt.)
You use я был if you are male and я была if you are female, было is neuter.
''В'' can be either prepositional or accusative and you have to remember which one it is in which case. In this case it is apparently accusative. время belongs to a small group of words that are neuter gender -> accusative form = nominative form