Iphone usage awareness
In iOS 9 or newer, you can head into Settings and then Battery on their iPhones for a list of all the apps that are using the students' batteries. Tap anywhere on the list of apps that appears in settings there to reveal exactly how long each app has been open in the last 24 hours or the last 7 days.

No need to install an app, no need to configure anything. Make sure you go into battery settings once on each phone before your trials begin so that it starts recording battery usage data per-app.