There is no law about when you are considered full-time or part-time. There are no rules about workers being full-time after a certain number of hours.
Bosses can treat groups of workers differently. They can give benefits to one group of workers that they call "full-time" even if another group that's called "part-time" works just as many hours.
No matter what you are called, all workers have the same rights. Even if you are called part-time, you have a right to overtime. Some laws only cover workers after they work a certain number of hours -- not how many hours you are scheduled, but how many hours you really work.
If a boss makes the groups so that workers who have legal protection against discrimination get treated worse, you may be able to make a federal or state discrimination charge.