|There's no trick to this. It's just the same as if you weren't using Survivor Algebra and tribes.
Each student takes his/her own test... and gets his/her own grade... But, call them "challenges" instead because it seems to take some of the sting out.
Whether you're using Survivor Algebra or not, I highly, HIGHLY recommend that you do NOT use multiple choice exams. I strongly feel that the old-fashioned question and free-response format is the best.
1) The most obvious reason we all know -- Students can just guess! Especially for problems like factoring. Multiple choice just doesn't work well. Students can often work backwards from the answers. Frankly, the only multiple-choice algebra questions I've seen that you can't work backwards from are just unreasonably difficult and tricky. And, in my opinion, there's just no need for that.
2) Students do not learn to write their work out properly. I can always tell when a student's previous teacher has given multiple choice exams... The student just doesn't have the proper training in showing his/her work -- the notation is always off too. Showing their math work the right way is great training for writing research papers later in their college careers and writing reports later in their working future. It teaches them important communication skills and it also trains them to have a more efficient way to approach problem solving in all areas.
3) There's no partial credit with multiple choice exams and that's just brutal to the students. If they know how to do the algebra but, drop a negative sign due to a case of nerves, should they really get NO points for that problem? Many disagree with me, but I don't think they should eat a zero for that. The grading (and partial credit) process should be very human. These kids aren't machines and they shouldn't be graded by a machine!
4) Here's THE most important reason of all: YOU will learn more about teaching math by seeing the student's work (and MISTAKES) than any other thing you can do. By seeing their thought processes, their notation and, most importantly, their mistakes, you will see what YOU need to do to explain things better. There have been so many times that I've been grading a set of exams and kept seeing the same error over and over again... This was a major flag to me that I needed to explain that thing better -- and even talk about this common mistake with the students so they can avoid it. Seeing these things over the years will hone your skills and make you a much more effective teacher.
Some other thoughts on challenges:
I usually keep them to just ten questions. Two reasons: I have to grade them by hand and it gives the students enough thinking time. You don't have to ask them every little thing... Just the threat that you MAY ask them everything is enough to get them to learn everything. (Hopefully!) So, I just hit the most important stuff -- the highlights.
I also ask them questions where they have to explain things... Like "Explain and execute the BEST method for graphing y = -2/3x +5." A question like this covers the process of graphing using the y=mx+b method, but the "explain" part let's me know if they understand the concept of slope AND that they aren't just plotting some points and hoping for the best. It's also great communications practice for them.
I'm also a big fan of handing out practice challenges. Why not give them the best chance of doing well? They still have to learn the stuff, so why not let them know what's really important? This also greatly reduces test anxiety.
If you don't like practices tests, then how about handing out an outline of important items and key points to focus on for studying? You can even work on this during your question/lecture time. Again, this really decreases anxiety.
I'll hit on some more related items in my section about trust.