If you really want to create a custom bitcoin application, forking from an existing app should reduce reinventing the wheel and can act as a learning resource. For example, here are two open-source Android bitcoin apps whose source you could grab:
The simplest solution that would serve a
bank=>bitcoin=>coffee arrangement would be only implementing a
bitcoin=>coffee app for students; and a
bitcoin=>coffee notification app for the cafe.
Save yourself the headache of handling fiat money in the app by either leaving the conversion of fiat currency to other more generic unbranded Android bitcoin apps or integrating an existing bitcoin provider into the app (e.g. Coinbase).
Please note that Android is not a particularly secure platform for bitcoin wallets - this is simply because Android by-design allows users to have true control over their phone; but most users will never lock down their phone and apply security hygiene to the necessary extent*. If you go forward with this, the main Stack Overflow site could also prove useful.
* Apple users shouldn't feel too smug :-) iPhone are locked down for monopoly control. User security is a convenient side-effect.