Kivy Multiple Screens KV file

MainScreen:

<MainScreen>:
	BoxLayout:
		orientation: "vertical"
		Label:
			text: "Plan lekcji Biskupiaka"
		Button:
			text: "Plan Lekcji"
		Button:
			text: "Dzwonki"
		Button:
			text: "Przerwy"
		Button:
			text: "Nauczyciele"
		Button:
			text: "Wydarzenia"
		Button:
			text: "O programie"
			on_press: root.manager.current = 'about'
		Button:
			text: "Wyjście"
			on_press: exit()
		Label:
			text: "Copyright (2014) Narzew"

<AboutScreen>:
	BoxLayout:
		orientation: "vertical"
		Label:
			text: "App created by Narzew"
		Button:
			text: "Powrót"
			on_press: root.manager.current = 'main'