Use app without google play? I want to develop an app for my class. Can that app work on my friends aka class mates device without uploading it on the google play.If possible then how?
Yeah it is Possible.

I think you are developing your app using Android Studio. In Android Studio go to the option Build -> Build APK for building your app. Then you can share your app with your friends. But Android requires that all APKs be digitally signed with a certificate before they can be installed. So you need to sign your app.
Here is a tutorial for building signed app. Then Click on Build -> Build signed APK for building your app.