It's better to pick up the phone when you call, as long as there is a reason, it won't be difficult for you, and not picking up the phone will lead to speculation.
Resignation is not as serious as you think, it's a common occurrence in KFC.
I think that if you didn't agree to work at least one day a week when you went in, you don't have to quit now. Maybe you'll change your mind later, unless you really don't like the job, but KFC work is pretty exhausting.
I don't know how far your school is from KFC, but if it's far enough, just call the manager and tell him you want to quit, and then return the uniforms, name tags, and employee handbook later. Like leaving the communication form they will be able to deal with, do not care what the agreement, will not have any impact on you. As long as the reason is sufficient, they won't say it's difficult for you. The first thing you need to do is to get out of the office and get out of the house.