Credit to HongKiat.com's blog article here for directions on creating a bootable Mac OS on a Thumbdrive, which allows for installation of the OS when there is no recovery partition available.
http://www.hongkiat.com/blog/clean-install-mavericks/
Their directions appear to work for both 10.10 and 10.11 as well, with very few modifications.
In a nutshell, you download the installer on a working computer through the App store, save it to the desktop, format a thumbdrive as OSX journaled, install to the thumbdrive, and then boot the target computers from the thumbdrive.
Here are more specific steps. If you need screen shots, the link above offers really great step by step guidance.
Download the installer on a working computer through the App store and prior to letting the installer run (it deletes itself after finishing), you copy the installer from the Applications folder, where it is downloaded, to the desktop.
You then format a USB thumbdrive with enough capacity for the OS, usually at least 7 GB, as Mac OS X Journaled, which you can do using Disk Utility.
Then you use the Terminal command line to do an installation from the installer. The syntax looks like this:
user# /pathtoinstaller/installer name.app/Contents/Resources/createinstallmedia --volume /Volumes/nameofthumbdrive --applicationpath / /pathtoinstaller/installer name.app --nointeraction
An easy way to get the paths right is to find the files/folders/volumes in the finder, and to drag them into the Terminal window as you type the command, it'll automatically add any characters needed for things like spaces in filenames, etc.
Once you've created the USB drive with the installer files on it, you can boot from it. Plug it into the target Mac, restart it, and hold down the option key as it turns on. It should boot up into a screen that lets you choose the startup disk. The USB drive should show up with Install Mac OS X as the name, and you can click it.
The bootable Thumbdrive will have some utilities built into it that let you format the disk if needed, as well as running Disk Utility for "First aid" on your drive, as well as repair permissions. The default installation is not a "clean" install, so it upgrades the existing OS without disrupting end user files, however, I'd always suggest doing a backup of the user's data before doing this.
