They don't let us practice injections on each other at SB because I guess something not too good happened years back. They also don't have us practice NO2 on each other
For injections they blunt the needle and we place it on the mucosa in the correct spot. The faculty checks that the angulation and positioning is good and that's our competency. Of course the faculty is more than willing to hold our hands for our first couple injections on real patients.
Otherwise we practice perio probing, scaling, and ultrasonic technique on each other (AKA free crappy cleaning during class). As wigglytooth said, we take impressions of each other at the end of first year, and we take ortho impressions of each other during second year.
All other dental work is done through a classmate or a student in a different year.