SB, I think the way suzywong worded that was just right. Maybe it is along the line you should do it, not too much information, but it says what it needs to.
At the end of the day, you know your parents best. You know i have told you before, it is OK for lucky people like myself who have the best parents, but you have to do what is right for you. And if that means not telling the whole truth, then so be it.
When all is said and done, you need to say something soon, because it will be better to hear it from you, as if you don't tell them soon, i am sure someone else will.