That's the point of the reality check, to do it in a dream and proove yourself that you are dreaming. It's ok to do RCs when you are awake and you probably should, because when you're in a dream you think you're awake.
I don't know what mp3 you used. There are lots of them on the internet, you should find out how you can use that one in particular.
The images were probably HI or hypnagogic images, it happens when you are between a waking and sleeping state. I don't know how a Trance induced lucid dream works. It sounds like WILD, but I really don't know. I also don't know why trances could be dangeorus. I mean as I understand we are in them a lot, while we read and do stuff like that. Ask the teacher to be specific?

Hope I helped.