If you want to try and practise all your spells, go to the place where you help that photographer get his camara back off the roof. Then you will see a lot of Slytherin kids all over the courtyard, then you will see Draco Malfoy, press and hold R1 until your aiming at him. Then produce a charm and you will go into a fight with him. Don't worry he's easy to beat, but you can fight him as many times as you like just keep doing the above and he will keep fighting you! So you can practise all the Defensive Spells that you are not very good at using, until you feel comfortable with all the spells.

Hope This Helps!