To dream of old classmates implies that someone from your past is relevant in understanding a present friendship or association. Some inner realization from earlier days will help you to resolve an issue today.
If you dream of your current classmates, they represent aspects of your own personality. Look to the people you dreamed about for signals of personality traits they have that you desire in yourself, or that you wish to get rid of.
If you dream of former classmates (from years gone by), this suggests that you need to draw on your old associations with your former classmates to gain insight in some current relationship. You learned a lesson from your relationships with the person or people you were dreaming about, and that lesson can help you in some aspect of your life now. So look to the people you dreamed of, and the experiences you had with them, for clues to how to act in a current situation or relationship.