Code-based linear and nonlinear seismic assessment procedures and P25 preliminary assessment technique, a recently suggested method to identify the collapse vulnerable RC buildings are summarized. Then, an existing RC school building which has many structural deficiencies has been investigated for the required performance levels. Damage levels of the structural members and the seismic performance level of the structure have been determined using linear and non-linear approaches. P25 PreliminaryAssessment Method is also applied to the same structure and the computed P25 score is found to indicate that the building is collapse vulnerable, in correlation with the detailed analytical approach. In order to examine the effect of each inadequacy on the whole seismic behavior of the building, a parametric study has also been performed by decreasing and increasing the level of the deficiency. The change of the maximum base shear values for each case has been compared and discussed.