You need to go back to the doctor and try something else. There are many different oral antibiotics for acne (minocycline, oxytetracycline, erythromycin, doxycycline, lymecycline) and if one hasn't worked, another will. These need to be taken for at least 3 months to clear skin, so don't give up too soon.
In addition to oral antibiotics your doctor can prescribe a topical retinoid called Differin cream which contains adapalene 0.1%
Another option, if you have started your periods, is a special contraceptive pill called Yasmin. This is commonly prescribed to teenage girls for hormonal acne, not just for birth control.
If this doesn't work, your GP can refer you to a Dermatologist at your local hospital for stronger medication which regular doctor can't prescribe.
As you are under 16, the NHS will pay for your treatment so do go back to your doctor.
Best Wishes, Eloise.