If and when you decide to talk to the teacher about this, keep in mind you heard this information via your son who spoke to his friend who says such and such happened.

I'm a teacher and mom of 3 sons, so I've seen it all from both sides. You really can't be certain what you heard about what the teacher said is completely accurate. I'm not saying anyone would intentionally lie; I'm just saying sometimes things get a bit mixed up in translation. So, approach it with this is what was reported to you about your son's absence, and that if that was what happended she should discuss any concerns about your son's absence directly with you rather than with her students. I'd kind of leave it at that at this point. I wouldn't go as far as to call it bullying, but it would certainly be inappropriate for a teacher to discuss a student with other students, particularly when they aren't present. I don't even let my students discuss other students who are not in the room.

I feel your pain about the running thing, but that really isn't a new fitness goal. I clearly remember running a mile each and every day when I was in 5th grade forty years ago. Hated every minute of it. All three of my sons did mile runs in 5th grade. We all live through it, but yes, it was miserable every step of the way.