Free for a day Apps - legal for use in Apple Configurator?

Today, several **paid** apps we have our eyes on to purchase for use on our classroom iPads through the VPP program are available **free** for the day, so I downloaded them and installed them in Apple Configurator, where they indeed show up as being free in the list of apps and can be installed on iDevices without problems. I'm concerned that somehow this is not legal to do, but I cannot find any verbiage indicating this illegality. I'm assuming that if the app is free at the time of download, it remains free to install on devices despite the app's previous, and future, availability as a paid app. Does anybody have any definitive information?