In that case; using a free app environment will do the trick in most cases.
Unless you have a specific case where the free app limitations are blocking you. But for education/demo's I don't see these directly.
i need to deploy my mendix app on private server is that possible ? and how to please ?
But then you need a license key, since you don't want to pay, that's not an option.
Mendix has a licensed subscription model, so that you can use the platform for free. Until the moment you need to deploy an (finished) app which adds value to the business. Seems fair to me.
If its for true educational purposes; https://www.mendix.com/university-program