First, in regard to correcting the teacher previously for the misspelling: good for you. Your daughter needs to see her name spelled correctly on labels because she is learning it.
Second, yes, you have a two-part solution on your hands.
First, you should certainly encourage your daughter to stand up for herself by telling the boy "Don't spit at me. I don't like it." in a strong voice. I don't believe in letting little ones this age have to 'sort it out', however, doing some role-playing with puppets or dolls can help her get outside the situation a bit and find that voice she needs to tell him *back the hell up, mister*. I find that my preschoolers who can use that strong voice get good results, both because it shows the offending child that they aren't going to accept being treated this way AND because that voice gets a teacher's attention pretty quick, without 'tattling'.
Here's the second part: tell that teacher, and have the teacher *tell your daughter* that it's okay to interrupt circle time if someone is hurting or spitting at her. (Another favorite preschool teacher once told me: "in the world of adults, spitting is the same as hitting, it's still considered battery, legally"). If your daughter has explicit permission from the teacher--in person-- to speak up and defend herself, that's important. The whole idea of circle and gathering times is both community and instruction; kids can't learn when they are worried about what that other child might do. If it were happening within my group, I would want to know. This isn't about you being "that parent", it's about little Johnny not being allowed to be disgusting and offensive at school. By talking to the teacher, you are also helping the boy learn how to be at preschool.
Speak up, Mama! I hope it all works out.:)