Missing characters

I'm a bit confused. Waterloo Road had so many characters disappear without a reason of some sort. What happened to Lewis Seddon? Which, I don't care about him. What happened to Dom Hammond? He was last seen season 3 episode 20 and just vanished without a trace without a reason. A lot of characters had that. Leaving without a reason and leaving unexpectedly. I just don't get why people leave and they not give a reason. They are just forgotten about.