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?