My boss fired the 2 people I worked with and now I’m by myself

I work as a receptionist in a medical office. There were 3 of us. Yesterday, he fired the other two and now it's just me. I was working part time because I'm in school and now he wants me to work full time to cover for them. I want to quit because the office was already a mess and now all the responsibility and stress is going to fall on me.