Upper room ultraviolet germicidal irradiance (UVGI) has been shown to reduce the concentration of bioaerosols in controlled chambers. However, there is a lack of experimental results on the reduction of bioaerosol concentrations by UVGI devices in actual uncontrolled buildings. This study was carried out in an American elementary school in the Midwest. Two sampling procedures were carried out in six selected classrooms with similar dimensions that were separated into two groups: (1) UVGI exposure group and (2) non-UVGI control group. Two-stage Tisch culturable impactors were utilized to collect airborne culturable bacteria and fungi. Monthly samples were collected during unoccupied period in sampling Procedure A and during close-to-occupied periods in sampling Procedure B. Student absenteeism data were collected. Nonparametric statistical methods were applied. Neither analysis of microorganisms nor student absenteeism showed a significant difference between the UVGI exposure and non-UVGI control groups in Procedure A. Analysis of the airborne culturable fine and total bacteria levels (1–8 µm) was significantly lower in the exposure classroom than those of the control classroom using Procedure B ( P values < 0.05). The result indicates that collecting airborne bacteria close to occupied time could be more effective in evaluating the performance of upper room UVGI. In this case study, upper room UVGI can reduce culturable bioaerosols in a crowed environment like classrooms.