Once you've started a class, click the 'Award multiple points' button in the top left hand corner.
You will notice a new list of options that are available on the right of the screen. To select all the pupils, simply click the 'Toggle All' button and this will select all of the pupils in your class.
You can deselect pupils if needed by clicking on them.
Now that you've selected all the pupils you want, you can reward the toggled pupils instead of rewarding each pupil 1 by 1. The 'Choose award' button allows you to give all selected pupils a behaviour of your choice.