Fix problems with the app

This commit fixes three problems with the application:
- statement global lines is an error, it does nothing
- two students don't have a focus and cols has only length of 4 so the
app dies when it picks a wrong student
- chooseStudents checks that it finds three different students, but it
may return multiple same student. Fixed by using a function in python's
random library
Merge pull request #1 from lamperi/master

Fix problems with the app