I went to college full-time from February to May and then dropped out on the first day of classes in September. Am I considered a full-time student?

It depends.  You must decide based on the definition provided by the IRS.

A full-time student is a student who is enrolled for the number of hours or courses that the school considers to be full-time attendance.

To qualify as a student, the person must be, during some part of each of any five calendar months of the year:

1. A full-time student at a school that has a regular teaching staff, course of study, and a regularly enrolled student body at the school, or
2. A student taking a full-time, on-farm training course given by a school described in (1), or by a state, county, or local government agency.

The five calendar months do not have to be consecutive.

This information is found in the Personal Exemptions and Dependents chapter of Publication 17, Your Federal Income Tax.