Schilke Standard Large Shank Trombone Mouthpiece 51D - This would typically be more for a euphonium player, but will work for trombone. It will probably take the edge off of the typical trombone sound. The cup is deep-ish, but the rim is a bit narrow and has a lot of bite to it.
Bach 6.5 AL Large Shank Mouthpiece - This is the size your beginners start on when they are 6th graders. Unless your students didn't grow much since they started, this will probably be too small. The rim will be comparable in size to the 51D (I don't have the exact measurement in front of me) and the cup will be quite shallow. You will not get a very rich sound out of this mouthpiece as it will not utilize the entire potential of the embouchure.
Bach 5G or 3G - These would be your best bet if you're looking for a "one size fits most" mouthpiece for high school sized/aged students. It will be big/deep enough for them to reach a higher potential for tone quality, air usage, etc etc.