Wi-Fi won't stay off

I have a Motorola Droid and have just installed the latest update. The Wi-Fi network at my workplace is temperamental, so I prefer to simply turn off the Wi-Fi and use 4G. The issue is, when I turn it off (in Settings ~> Wi-Fi ~> move the slide bar to "off") it shows as off, however the next time I turn the screen on, the Wi-Fi is back on. How can I stop this?