Self defense skills are practiced every time you go to your tae kwon do school for training

Every punch, every kick, every yell is self defense practice. You might not realize it, but the simple techniques that you practice in class, increase your confidence, control and ability to defend yourself if the need should ever arise. 

You learn throughout your training where the vital points on the human anatomy are. These are the targets you use to perfect your techniques. Learning these points comes with form practice, because each strike, punch or kick is aimed at one of these points.

So if you never specifically take a "self-defense" class, your tae kwon do training has prepared you. It will become part of your sub-conscious.

You will probably start by memorizing techniques. But consistent attendance in class and practice of the techniques will soon lead to developing effective responses to whatever situation you find yourself in.

How do you keep from getting mugged? Practice, practice, practice.