"There are five other students in the school, but they are sick."
Translation:Estas kvin aliaj lernejanoj en la lernejo, sed ili estas malsanaj.
Lernejano = student at a k-12 school -- usually under 18 years old.
Lernanto = anybody who is learning.
Lernanto is more general, but if I were grading it, I would accept it in this context.
Keep in mind that "studi" generally means "to have a your major" and "studento" is usually "university student."