Solution:

1. First, open the homepage of your school's NMC website.

2. Then find the "self-help" option on the page, find it and click.

3. Enter the account you usually use for surfing the Internet in the page that appears.

4. There will be a list on the left side of the interface. Locate the item "self-service hall" and click on it.

5. A sub-list will appear under "Self-service User List". Find "Online User List" and click again.

6. Then you just need to click the "Clear Online Information" option on the right.