The appropriate capacity of a ventilation system based on the air infiltration rate in Korean classrooms is investigated to obtain optimal design conditions for ventilation systems. Theoretical and the experimental analyses are performed to estimate the proper ventilation capacity with a consideration of the air infiltration, the indoor air quality, and the ventilation rate. The air infiltration rate of the classroom is measured within the range of 0.5-1.5 1/h, and the required ventilation rate should be decided not by the contaminants (Formaldehyde and TVOC) emitted from the construction materials but by the carbon dioxide $(CO_2)$ emitted from human breath. The appropriate capacity of the ventilation system based on the air infiltration rate of the classroom for elementary schools is 500CMH and for middle and high schools is 800 CMH. The measured and the estimated values of $CO_2$ concentrations are very similar and the modeling equation of $CO_2$ concentration can be used as a reference for the proper estimation of ventilation rate in Korean schools.