Does full time student only refer to college or can it also mean high school?

Full time student can refer to either high school or college.  Full time student for the purpose of taxes means that the student was enrolled in a school for at least one day of at least five months of the year.  It is not based on the course load of the school.

The Form 1098-T is issued by educational institutions showing the amount of tuition that was paid during the year, as well as other information about the student's enrollment in the school.  It is typically issued by colleges, not high schools.

@sonorat123

Why do you ask?
Being a full time student , between the ages of 19 and 23, is one of the tests (but not the only test) for the student to be his parent's "Qualifying Child" dependent.  It does not matter if it's high school, college or trade school.