Apple App Store is where all the apps for apple iOS devices are located.
App Store is available on iOS Devices such as iTouch, iPhone and iPads it is also available on Mac computers running Mac OS 10.6 or later, Mac OS codenames: Snow Leopard, Lion & Mountain Lion.
Most Apple ID's have a credit card bound to them. When dealing with classsroom Apple ID it is best to create a free Apple ID account. There are plenty of free educational apps. You can also use iTunes gift cards to purchase apps. There is also an Educational Volume licensing program to purchase 20 or more of the same educational app at 50% off.
Apps like AppShopper are helpful as they alert app prices changes and offer paid apps for free.
Apple App Store Link.