Windows 7 : Any way to disable "show caracter" in WIFI network properties?

As far as I know, you cannot override this option.

You can, however, set up a standard user (non-admin) account for the users and set up the network on that account. From there, they'll need the admin password to be able to check the 'Show password' box. Note the 'Admin' logo next to the check box below:

Wireless keys are usually not very secure, so using it as the only mechanism of controlling access to a school's wireless network is not an ideal solution. You should instead keep track of the Wireless MAC address of each laptop and use access points that allow you to restrict access via MAC address. There are lots of WAP Management solutions available that let you control access to a large wireless network. Alternatively you could setup the DHCP server to only respond to MAC addresses that have been validated, which would help prevent unauthorized users from accessing the network.