[quote=Brovo]\Note: Extreme mental stress or disorders like depression =/= faking it. Not wanting to go and thus simply... Not going? Faking it.[/quote]

I'm pretty sure this went without saying.
It's basically the same as going "Having the Flu, Needing Surgery, Lost a limb = Not faking it".

I mean no shit you're not faking it. May it be mental or physical it counts as a legit issue. :P
However this is all assuming it's a case where the person actually has depression, and isn't just claiming they are for attention, to dodge a bullet, avoid taking blame for something etc.

[quote=Neobullseye]Your teacher/boss will not only be distrusting towards you, but also towards anyone else with the same excuse, even if it's actually legit for them.[/quote]

Eh... Maybe my High School was just an exception, but schools are pretty poor at tracking what's legit or not.
As long as a call was made (especially if you had your parent make the call) they often never thought twice about it.

In fact, the only time I was accused to faking sickness was when it was legit. 
I would used to get very bad headaches in the mornings (Technically still do. Nothing ever worked in treated it. My best guess is that I simply adapted to signal out the rather constant pain, it still will pop up time to time, mainly when I think about it... like now... but still mostly unnoticeable. :P) and this would always cause me to slow down in my daily routine enough to be late for class. My lateness was so common from this, that even with medical notes and parental calls the teacher being an ass just assumed I was being lazy and using it as an excuse. This may of been partially because I was never the best at the class to start with so he was more disposed to assumed I just didn't care.

Basically, I think it's more just the matter of the teacher you get and their opinion of you.

It could be totally fake but if the teacher likes you they'll probably let it pass and assume it was legit. Or simply not care as long as you do well, I've had teachers who were like that as well. But your reasons could be 100% legit and the teacher may still accuse it as fake for no reason other than that they just don't like you.