To me, this does not make sense. Other than going to the same elementary school, Ada and Sofi would not be in the same middle (or junior high) schools due to the age difference. Even if they went to the same elementary school, there is very little intermixing of students between grades at the elementary level, so the story is lacking as to why they are such good friends. This creates a credibility problem with the age difference. In order for Ada and Sofi to be such good friends, there would need to be either (1) serious backstory to explain why this is so, given the fact that they would be in different schools, or (2) a closer age between the two. Putting Sofi at age 15 explains why she could (conceivably) have a job at the mall, however, this means that Ada would need to be bumped up a little to be closer in age to Sofi so they could have been in the same school at the same time. Without a backstory (went to elementary school together, neighbors, etc.) the story doesn't make sense this way.