After exiting the main building, before going to the dorm you are free to explore the area outside, if you walk in a straight line from the door you come out of you will end up by a road, and you should see a group of guys hanging out skating and sitting around. once you talk to one of them and convince him how 'cool' you are, he asks if you want to see a "nose-slide" or a "tre-flip". select tre-flip and after a short cutscene, take advantage of another's misfortune (like a true photographer) by snapping a picture with
, your achievement soon follows.
Also be warned that doing this MAY have consequences in a future episode.
Can confirm that if you miss it, coming back to get it through 'Collectibles Mode' will still count towards: