What sort of professional boundary should I set with other students when I am a grader for a class I will be taking? Next semester, I am going to take a course in which I have some background knowledge. Given the relevance of the course in my research, the lack of available students with sufficient knowledge in the course (it is one that is rarely taught at this school, and most of my supervisor's students either graduated last semester, or have just started with him this semester), and my general need to find some courses to TA for, the professor (my supervisor) had offered me the grader position for the course. The professor would be grading my own assignment, and I would then be marking other people's assignments based on those. It appears this situation has occured and been asked about before on this stack exchange, but not from the point of view of the TA. 
I initially expressed my discomfort with this situation, but, upon being told (by my professor) that this had been done before, decided to accept, to maintain a good relationship with my supervisor. 

I however have a number of issues which, as the beginning of next semester, are bothering me more and more:

I know a number of the other students in the class, and we often used to organise study groups and work on the assignments together. Would it be ethical of me to still participate in these study groups ? On one hand, I will not have access to the solutions until we submit our assignment, on the other, I will definitely be able to see if someone makes an obvious mistake in their proof that I, as a TA, would deduct points on.

I was initially thinking that the best course of action for me would be to cut off all relations related to the class (that is, remain friends with the students I know, but not bring up the class with them), but at the same time, I would thus be depriving them of help that I would have given to them as TA had I not been in such a situation. I'm finding it extremely difficult to set the line between "We are fellow students together and helping each other out" and "I am your TA and you are a student, and the help I am providing here is so you can properly learn the class".

Updates in responses to questions:


I have asked the professor for clarification; he says this is not the first time he has done this
I could certainly ask for "blind" grading
I am taking the course as a student, but have some background in the course
Actually, I think the best course of action would be for you to decline this offer. You have different personal relationships with different people in the class and it will be difficult to put aside all of your unconscious feelings and grade in a completely unbiased manner. It also opens the possibility of your friends putting pressure on you, which you can resist, but which could become uncomfortable. 

An exception would be for grading that is purely objective, not requiring any analysis or judgement of the quality of the answers, such as mechanically recording the answers in multiple choice questions. 

However, if blind grading could be arranged so that you grade paper not knowing who wrote them, then my reservations would probably disappear. 

Blind grading would leave you free to interact normally with study groups and would lessen or remove any pressure that could be applied against you. 

At a minimum bring up such concerns with the professor and point out the uncomfortable position it puts you in. 



Perhaps there is a possibility that you help out the prof by grading a different class and suggesting you grade this one in a future offering.