Hi, thanks for your query.
After going through the information you have provided it seems that you have tinea versicolor
in which there is a non itchy scaly rash
chest , upper arms, shoulders, back.
It is a fungal infection
and is seen in adolescent age group.
Oral anti fungal tablets and local anti fungal creams. Treatment usually eliminates the fungal infection. However, the discoloration of the skin may take up to several months to resolve.
Avoid applying moisturizers ad wear loose cotton clothing.
You may contact a dermatologist
for exact diagnosis and treatment.
Hope this helps you.