Can you recommend some good design for a web based quiz? [on hold]

I'm building a quiz for a customer that wants to teach chemistry and Biology to their pupils.

The app will deliver content via the quiz. Its structure is simple. User's pick which subject they want to learn and then questions will be selected from each sub-topic and displayed. What I want to do is find a way to keep the users engaged. What can I include to keep users engaged? If you can recommend some design and gamification ideas, that'd be appreciated

Thanks