Should I decline a college ring as a present?

Well for the time being you can kind of fib and say that a college ring may not be a great idea now since you're going back so technically you're not finished with school. By that time you may have a change of heart. If not then just kindly accept it and thank them. Besides, they're grandparents...if it makes them happy to go out and purchase you a nice piece of jewelry then let them because that also falls into the grandparents spoiling grandchildren clause.