It is always surprising to have pimples which may be caused due to external factors. There are efficient ways to get rid of acne.
Back and Shoulders
The cause of back acne would be backpack straps or tightly fitted synthetic clothes. The fatty food and fried food may cause acne in the body.
You can wash your body with saltwater or wash with apple cider vinegar and water in 1:2 ratio.
A tight collar may cause pimples. Neck acne may happen due to reaction to shampoo or hair rinse. If you have passion for sweets then it is the cause of pimples.
You can wipe your neck with cotton pad drenched in lemon or lime juice then rest it for 10 minutes and then wash it off with warm water.