Actually, it was not hard to understand the matter.
Bai Lin had indeed only attended a regular elementary school for a few days, which was when she was brought back to City A by the Bai family. Other than that, the name "Bai Lin" had not left any information about her admission to schools.
No one had asked about Bai Lin's whereabouts for many years. Granny had always refused to meet anyone in the countryside too. Those who wanted to inquire about her whereabouts were all swept away by Granny with a sweep of her broom.
From their point of view, investigating Ella and Bai Lin were two completely unrelated matters.
