You can only hide the Wi-Fi password from non-administrative users.
1. Login as the user you want to hide from and remove the password from Keychain Access.
2. Login as an administrator and connect to the Wi-Fi network in question.
3. Open Keychain Access and drag the Wi-Fi password to the System chain.
All users will authenticate to the Wi-Fi network using the password in the System chain. They will even be able to see the username, if applicable. What they will NOT be able to do is show the password without administrative credentials.