Like any other hobby, if you neglect your studies, job, family, etc. to the exclusion of your hobby, and make it the one center of your life, then yes, you can have serious problems. However, if balanced with your life as a whole, it can be a good form of social interaction, grammatical and communicative improvement, and if played for this effect, can also help people think strategically and how to overcome challenges.
One other thing - my grades never slipped before or after D&D - I started playing in the 5th grade, and never had any noticeable change.