I think it's pretty bad form for teachers to be allowing students to discipline each other. It will just be a case of "who don't I like today" and all students not understanding what's going on.
If the teacher really wanted to encourage the kids to 'keep watch' over each other, she should be doing it in a positive way, rather than the negative. IE - "so and so gets a sticker because blah blah said he was being nice to his friends in the playground". Encourage them to "out do" each other with kindness, not bully each other.
OP, I'd be speaking with the teacher and any aides your son may have. It's not fair to any kid to have a 'because I said so' type reaction thrown at them but especially one with autism IMO.