Lesson Items will last for a set number of lessons.
You cannot cancel the use of an item while it is active. It will automatically be deselected when its effect runs out.
Items you use will only be deducted from your inventory once a lesson begins. As long as the item effect is active, no further items will be deducted.
You can check how many remaining uses of an item you have on the Lesson Team screen and the post-lesson Results screen.