Our school does this as well, also randomly (as far as we can tell.) We just took it for what it was- a "buddy." I guess you have to pick your battles, but our buddies met for a brief time, once a week- they aren't expected to tutor each other, but to get to know a kid of a different age/grade. They made little things for each other, like cards and bookmarks, and when the chemistry was good, it was a really positive experience. We did talk about how to handle the reading (they often alternated, every other page, for example) but otherwise it wasn't worth complaining about.
That's exactly what it is in our school, except it much more occasional than that.
My breaking point in complaining is to look at how much of the instruction is being done through this way. My DS is in half day kindergarten. His teacher reports that when you take specials and recess and other extras, she has a total of 90 minutes of instruction per day, or just 7.5 hours a week. If something like this is going on for 30 minutes once a week
and my child is frustrated with the arrangement, then that's approaching 10% of instructional time not serving its intended purpose. That's beyond my break point and I'll bring it up with the teacher as a point of discussion.
