Is a college freshman who is 18 to old for in high-school that 15?

An 18-year-old college freshman would typically not be attending high school, which is designed for students aged 14 to 18. College freshmen are usually 18 or 19 years old, having graduated from high school the previous year. Therefore, it would be unusual for an 18-year-old college freshman to be in high school with 15-year-olds.