Download an Ubuntu package repository to a USB for installation on off-line system

I use a number of computers running Ubuntu for students; these are not on-line. When a new long-term release comes from Ubuntu, I create an iso on a usb so I can install the new release on the off-line systems. I also need to be able to install the gnucobol package from the ubuntu repositories but I want to be able to do it from a usb - not the apt-get install on-line method. I need help in knowing exactly what to do both to down-load the package and then to use that usb to install the package off-line. (The traditional method of updating an existing computer is too slow.)