@Hussam_el_din 

Hi, there is no option to add all client devices automatically into parental control device list, you could only add devices manually.

Maybe you could add all devices first and then setup Access Control Whitelist to add the trusted devices only, in this way, devices not in the list will not be able to connect the network.

Note: wireless client devices now usually support random MAC address, so make sure the students device disabling the random MAC address, or when it changes randomly, reconnecting the WiFi will not be possible since only specific MAC addresses are allowed.