This study aimed to standardize the BVR CCD photometry system of the Gwangju Science High School Observatory. We have observed Landolt standard stars using the CDK14 telescope and the STX-16803 CCD camera and carried out the data reduction by Maxim DL. As a result, we estimated the atmospheric extinction coefficients using the observation on August 29 and 31, 2024. By a standardization procedure, we calculated instrumental magnitudes and color indices outside the atmosphere. The instrumental magnitudes of celestial objects measured on the ground by the atmospheric correction were successfully transformed into standard magnitudes, with the standardization conversion formulas provided as following equations.                         The observational analysis revealed that atmospheric extinction coefficients vary depending on the observing conditions and that extinction increases at short wavelength. Additionally, the B filter exhibited significant second-order extinction effects due to color indices, suggesting the necessity of applying second-order atmospheric extinction coefficients for precise observations. This study suggests that Gwangju Science High School Observatory can serve as reliable resources for both educational and research purposes. Furthermore, this study demonstrates the potential for producing high-precision photometric data that meet international standards, even at a secondary school-level observatory. These findings are expected to contribute significantly to future astronomical education and research.