What are the chances of a 15 yo having breast cancer?

Virtually zero: With very rare exceptions breast cancer occurs in women older than 30 years and usually older than that. See this site for more information on the subject. http://www.ncbi.nlm.nih.gov/pubmedhealth/pmh0001911/.

Low: While not impossible, it would be very rare.

Near zero: Only a rare inheritable form could develop on such a patient and even their this would be very early. Most any new change or lump in the breast would be related to hormonal changes or infection or something benign.