Yes, this will negatively affect your GPA for sure, however, it may not be as adverse of an effect as you think. As a senior, all the classes you have already taken cushion your GPA already. You can do the math to see how much your GPA will drop:
Tally up how many HS classes you've taken, add four points for an A, three for a B, and two for a C. Add one to the point value if it is a weighted class like an AP, (add five points for an A and so forth)
Find the sum and divide by the total number of classes. This will be your weighted GPA.