This happened to me back in high school. I was 17 and I was on a three day school field trip. Since we were that old the teacher let us wander around the town in the evening, and there was this roadshow coming into town at the time so everyone was pretty hyped. My best friend and I, on the other hand, were pretty reclusive guys who didn't like partying, so we just hung around the dorms and played poker and whatnot. On the second evening the teacher called us and told us that a group of the girls were not picking up their phones at the roadshow and he wanted us to find them (so he practically sent us on a wild goose chase, but the guys was an asshole to begin with, so it was expected).
So, we went in, and by the time we got to the place it turned out that the girls were already on their way back. As such we decided to return to the dorms as well when three kids of our age or a little younger (and another ethnicity, though again, skin color and being an asshole rarely correlates) started tailing us. When they caught up with us they started with the typical mugger speak about being asking for a time and what my pal was carrying in his pack and such. So, these guys focused almost entirely on him, and then... He bolted like a jackrabbit and left me there with the three guys.
Now then, for the record, I am a thin and relatively short guy, however that day I was wearing a leather jacket that arguably made my frame look wider and was pretty pissed at the teacher already, and then my best friend also just left me there like that. Needless to say, I was pretty cross and it probably showed on my face (which is apparently a little scary by default, or so I am told). Also, while I had no real experience with muggers (since I was a village boy), I had plenty of experience with wild fucking dogs, and I decided to use the same treatment as I did with those: straighten myself and walk in a confident stride without making prolonged eye-contact.
Needless to say the three guys had no idea what to do with me, so at first they started asking me for the time, then told me that they were only joking and that I should call my friend on the phone, at which point I told them to fuck off and that I am not so stupid to take my valuables with me after dark. At this point they really didn't know what to do and they started acting all buddy-buddy with me and tried to befriend me, asking me not to tell this anyone and that they were only messing around. In the end they said goodbye and left at an intersection without even as much as touching me.
And the icing on the cake? Past about 200 meters of that intersection I found my friend... collapsed in the ditch beside the road because he got a horrible cramp in his leg from running too hard and couldn't even get in his feet. And then the police showed up and they refused to take my description of the three guys because I told them not to substract 20 centimeters (0.6 foot) from their height, for which they answered that it was standard procedure because "I was very afraid at the time and being afraid makes them look bigger". Then I asked him if he really believed that we were attacked by 1.2 meters (~4 feet) tall midgets, at which point the patrolman told me to shut up and sent me away to ask my pal instead.
So yeah, that was one weird way to end a school outing... -.-