What are the suggested architectures and design patterns to be followed for constantly evolving Product Application?

Nowadays I am designing a new Application which is a school management system, which will have both web and android interfaces. I want to give this software application as a service. Though, we have gathered the requirements on our own and it is pretty sure that we will have to add many more features to our product by the time.

I want you suggestions that what should be the architecture and design pattern should I follow for this application. As you know that choosing correct patterns at the time of designing makes you product more flexible and friendly in all terms.