When in one of the few battles with Hogwart's students (just the Slytherin Gang, all others leave you alone), let them make the first shot and bounce it back. Just as you do, use your Flapendo with a quarter to half second charge to knock them back, thus trapping them with two spells to knock back, which they can never manage. Note: Beware, just as you finish casting Flapendo (depending on distance), you will be forced to use your Expelliarmus almost instantly.
