How to lock a file on a flash drive

I have a DMG install file for a piece of software that the pupils at my school need to use, but I want to be able to control the access of who is allowed to install this software. I'm busy and have no time to individually make sure the boys do not copy the DMG file to their Macbooks. I have a flash drive and the idea is that I put the DMG into a file that is password protected so all I need to do is quickly plug it in, open the file using my password, run the DMG and exit the window so that you need the password to re-open the protected file. I can then leave the boy to install the file and return the flash drive to me with the knowledge that they will not be able to open the file and copy the DMG.

Usually I will go into Sharing and Permissions and change the permission to "Write-Only". However. It will not give me the option when I try to protect it within the flash drive. Why not? And is there any way you can get that option back? Or, are there any other ways I can protect the file? I'm also open to any other ideas you have to stop the boys copying the DMG.

Thanks

You could create a new, encrypted, DMG and copy the contents of the current one you have into that (if you permitted to do so).

This way you can enter the password, allow them to install but even if they do copy the DMG they'll never be able to open it with the password.

How would I create a new DMG?

Open Disk Utility, New Image . . .

I'm afraid my knowledge of Mac OSX is not that advanced...

I'll knock some instructions up when I get home

First thing to do is right click - get info on the existing disk image so you know what size to create the encrypted disk image (dmg)

Here's a good guide:
Mac 101 | Security: Create Encrypted DMG Disk Images for Sensative Data

Once you have the encrypted disk mounted and the existing dmg mounted you can Drag the contents on the existing image to the new encrypted image.