What is the street address and subdivision? Buford City, Lanier and North Gwinnett all serve this area of Gwinnett County, plus a few are being redistricted for next year, so an exact location would help in determining that. I work in that area and live right across the river in Forsyth, so I'd be happy to assist you if you are looking in the area. Good luck!
Isabel's right - you would need the exact address to be able to tell. 30518 is Buford, but it would need to be in the city to go to Buford High School. If it's outside the city limits it could be one of any of a couple of different high schools in the area. Whatever information you can provide will make it easier to answer the question.