Final answer:
Streptococcus pneumoniae is the most common cause of bacterial pneumonia in school-age children.
Step-by-step explanation:
The most common cause of bacterial pneumonia in school-age children (5 to 14 years of age) is Streptococcus pneumoniae. This gram-positive bacterium is commonly found in the respiratory tract and can cause inflammation and fluid accumulation in the lungs. It produces virulence factors like polysaccharide capsule and pneumolysin O that interfere with the immune response and damage host cells.