Have you ever experienced a traitor?

At an industrial arts school I attended, we had to do an exam but we were all displeased about the instructors performance in preparing us for the exam and the headmaster of the school did not take our complaints seriously. So all of us got together one afternoon and planned to “boycot” the exam by staying home that day. I would still go to school because I had to collect something from the headmaster so I told them that I would not be dressed in uniform to show that I was not there to sit the test. All of us agreed. When I came to school the day that we were supposed to “boycot” to my surprise I saw three people from our group; who were speaking out just as much as the rest of us at our rally and was in full agreement of the “boycot”, fully dressed in uniform waiting to sit the test. They looked at me and said nothing and I did not ask them anything. Have you all ever experienced something like this in the workplace or school just like I did? How do you feel about this? Are they right for doing that? Would you do that in a similar event?