I think your idea has potential but I have some concerns. I really don't want to come across as discouraging so, instead, let me pose some questions you need to think about based on my teaching and software development experience.
- How are people going to enter equations into your app? Even something as simple as an exponent is going to require you to create a custom button or syntax, e.g. x^2. Even when I'm teaching online, I discourage my students from using that kind of formatting-based approach since an implicit goal of the class is being able to communicate technical ideas effectively using standard methods and notation. You're also going to run into formatting issues with things that are customarily not done from top to bottom. For example, solving the equation $x^2 + 3x + 2=0$ means solving two smaller equations, $x + 2 = 0$ and $x + 1 = 0$. If I were writing this on paper, I would put the two solutions side by side but that's going to be hard to do in a text based environment.
- How are you going to handle privacy issues? I'm not sure how much of a concern this will be depending on the specifics of your application and your market but, at least at the college level, there are some pretty strict privacy regulations that we have to follow. I'm not sure how much they apply at the primary and secondary levels.
- How are you going to handle data storage? Is this going to be a send and forget thing or are you going to save the student's work and the feedback so that they can go back and review it later? Having a centralized source to save the work is going to make this a much bigger operation. A potentially simpler option would be to save the data on the individual devices but then you run into a potential problem with "my phone died" scenarios. I think I would be comfortable with writing the data to individual devices, possibly with a restore option that a teacher could use to send content back to a given student, but you should have a plan in place to handle restore requests since, sooner or later, you're going to get some.
Ultimately, I think you've got an interesting idea that could be beneficial to both sides. The kinds of problem that lend themselves to this sort of auto-feedback application are the procedural ones that students can drill on on their own time which frees up classroom time for more interesting scenarios.