You should say "at the scene" as Raquel explained in this context. "On the scene" is an idiom used more informally. e.g. "He appeared on the scene some time later when everyone had gone home." "She is back on the scene after being ill for a while". It means that someone was absent but is now part of the action If you want to talk about something which is happening in the world of e.g. entertainment, sport, politics, you can refer to a particular "scene" e.g. "The Rolling Stones have been on the rock scene since the 1960s." See more examples here: http://www.linguee.com/english-german/search?source=auto&query=on+the+rock+scene

In English, the word "on" is used more for when a person is standing on top of something. For example: "The girl is standing on the desk." There are, of course, other uses for on in English, but that one is the most common. "At" is used especially for location. "I was at the bank when the fire alarm went off." Another example would be: "I was at the park when it began to rain." "On" would not be appropriate for those cases. If you said, "I was on the bank when the fire alarm went off", that would make it sound like you were actually standing on top of the bank. So the correct word to use in the above sentence would be "at". :)