In recent chapters (as of Chapter 168) of Gakuen Alice, Mikan's memories of her time at the school are due to be erased because
she loses her abilities saving Natsume.
However, people outside Gakuen Alice are stated to know about Alices. For example, some people (such as Hotaru) have obtained contracts from companies related to her Alice.
Why, then, do Mikan's memories of the school need to be erased?
It is one of Alice Gakuen's school rules that any student who leaves without graduating from Alice Academy will have their memories erased. Furthermore, because Mikan's case is, according to Narumi, a special one, that she knows so much about the secret organizations regarding the academy, it is best for her memories to be erased instead of being held captive by those organizations. It is the best way to protect her from them.
