In the episode "The Frycook What Came from All That Space", it is proved that Zim once truly was an Irken Invader, as when the Control Brain re-encodes him, it shows his status as being an Invader before being changed to Fast Service Food Drone. This is also shown in the unfinished episode "The Trial", when it shows Zim's history. Zim tried to get out of his underground training classes with the help of Skoodge, so it is unknown how he managed to be allowed to take the test. However, what is known is that he passed the test on Devastis and became an Irken Elite, making his way to Invader class.