Back acne is one of those things that can show up in your teens and 20's, and trust me when I say I understand just how much you want it gone. I was cursed with back acne as well.
Let's start with what you're cleaning your body with. Chances are good you're using a shower gel or a bar soap. One easy step is to swap your current cleanser with one geared more towards clearing up acne.
Try Nature's Gate Organics Natural Results Body Wash with Salicylic Acid (compare prices) or Neutrogena Body Clear Body Wash Salicylic Acid Acne Treatment (buy direct). Use this cleanser once a day in the shower and you'll notice your acne clearing up.
Are you active in sports at school? It's important to hop in the shower right after practice if possible, to clean your skin and avoid clogging your pores. If that's not possible, stick some cleansing wipes in your bag for a quick fix. Also, make sure to change your shirt if it's sweaty.
Be careful with what lotions and creams you put on your back and your chest. Stick with a daily facial moisturizer meant for acne prone skin for all of the areas plagued by acne.
And lastly, be patient. Acne is unfortunately something that most of us have to go through, but more times than not, there will be an end. If you make sure you are making good choices when it comes to what you're putting on your back and keeping it clean, you'll see it improve.
If it gets really bad, or you experience a lot of cystic acne, let your mom or dad know that you'd like to visit a dermatologist. There are prescriptions that can and will help.