— Biggest tip I can offer: Give them premade assets to use. One of the startlingly time intensive things is to have assets to make the game with. I know construct has a few example assets, and http://www.opengameart.org has a lot more.
Also, one of the best things for a short class is to spend the instructional period cloning something that will give the students a good, rounded beginning in Construct. In my opinion, top down shooters and endless runners are pretty good starting points. I'd copy something exactly, just to make the concept clear for your students. Alien Invaders, Flappy Bird, as close to the original as possible so they don't have to worry about conceptualizing the project.
I definitely think leaving some of the class for experimentation is an awesome idea, just caution your students against overly ambitious ideas. Some pitfalls to avoid: Any kind of casino-esque game (Poker, etc) because you just don't have the time to do arrays, sorting, and random sorting, any kind of RPG, any kind of crafting system.
Also, Extra Credits has an AWESOME video series on making your first game that might be worth a watch, to help inform your lectures. I don't know if you should show the videos as a whole, as it might eat too much time.
Follow up advice- making the art assets first is possibly a bad idea, as the students won't know what art assets they need. If making the tablets available for the full session is possible, I'd just do that, otherwise I'd schedule the tablets for half way through the project, and just use different colored squares/circles until then.
Hope that helps!