I actually did notice the strangeness of this sentence, but I thought it contained a hidden meaning if you read between the lines. Reviewing the raws again:
「底辺だった私の成績は最終的に学年で7位になっていた」
with Google, DeepL, my own small knowledge of JP, and even ChatGPT, it actually does mean she ultimately ends as 7th place despite being at the bottom of her class. Her grades "stopped being at the bottom" suggests that she got better. Cecilia being 7th despite being at the bottom probably contains the hidden meaning that only 7 people were present to graduate and the cause of that was probably because of Alice and her experiments. Of course, this is just speculation, but that's what I think it means.