It has the makings of a great app! I really like where you're going.
I do, however, have a few points of criticism.
1) The content is rather basic: after taking one 7 question quiz at 300 difficulty and finishing it in 47 seconds with perfect marks, I tried one at 1000, also with seven questions. To my disappointment it consisted of nothing but some "non-basic" quadratic equations. I finished this one with perfect marks in 2 minutes and 28 seconds. Now, I'm not saying that we need complex vector calculus problems, but a little bit beyond mid-year Algebra I content would be much appreciated.
2) The app itself is a bit buggy: Now, seeing as this seems to be your first app, this isn't really criticism or complaining, it's just saying I hope it develops well. It never crashed, but it froze for about 10 seconds three times while I was using it.
This has the makings of a great app! I love the layout/look of your app, and I love the idea behind it. Can't wait to see further versions of it!