I travel with college age students and have had situations where they have gotten separated from the group. It is not a good feeling when you realize it has happened.
Since our hotels are pre-booked, I make business cards for each with the hotel name, address, and telephone number, and my cell number (it works abroad). That way, if the hotel doesn't happen to have one, we still do.
Since these are older students, the rule for subways and buses is to know the direction we are going, and the final destination and to meet there. Either the entire group, or one of the instructors will wait until all are there. This happened on a trip to London. During morning rush hour, one student didn't make it on the tube, but met up with us at the destination. For our situation, we don't do the next stop thing, because that can backfire with regular trains. We had a group of students and one instructor get on the wrong train in Switzerland. They got off at the next stop, but they had been on a local only train, and our next stop wasn't the same. Unknown to us,our train passed them by, and we met up several hours later in Zurich. We had SwissRail looking for them, but had no information until they arrived at the train station. That one was really scary.
Another thing is, if they are on their own for some free time, they need to know name, location, and look of the meeting place. We had two sitting at the wrong church in Vienna last May. It looked the same as the one that was our meeting place. Luckily they discovered their mistake and made it to us, just a little late.
A little time for preparation like this makes everyone's trip more enjoyable.