To make the function keys on Apple Keyboard work correctly in Ubuntu and its variants (Lubuntu, etc.), the following commands should be used to permanently set the corresponding "fnmode" property and update the configuration files:
>echo options hid_apple fnmode=2 | sudo tee -a /etc/modprobe.d/hid_apple.conf
>sudo update-initramfs -u -k all
You need to reboot after this for changes to take effect.
Reference: https://help.ubuntu.com/community/AppleKeyboard