In my med school experience, as an M1 you had interaction with M2s insofar as they were in charge of all of the events and clubs. You were assigned an M2 as a big brother/sister, and they were the ones you hit up for used books. There were also various non-year-specific ethics type lectures throughout the year where both M1 and M2 classes attended. During the clinical years, you might be a M3 while an M4 is a sub-I in the same rotation (or a year later vice versa) so you have interaction with them them. Additionally, if someone the year ahead drops back for a year/course for any reasons (especially if they missed a block because of pregnancy), or are asked to retake something, or for the PhD students who drop back into med school rotations after doing a year of PhD stuff, you will have upper classmen as classmates.
But in general, the M4s beat up the M1s for their lunch money and stuffed them into lockers just like high school. 🙂