The answer is varicella (chickenpox) in a teenager who had not been vaccinated.
Your diagnostic acumen was right on and I initially thought it might be dermatitis herpetiformis too, but Chickenpox it is. For more info on Varicella, check out my old post.
I am sorry for the delay with the answer. This disaster zone is what has been keeping me busy at EverythingHealth.
P.S. that is not me in