Hi BlackFeather, this..........."said that I'm not depressed because if I was, I wouldn't be able to function (go to school, etc.)" is not necessarily true!!! Not in the slightest!!!
Plenty of people who are going through depression (diagnosed or undiagnosed) can still function..........not as well as..........but can still function. Some can't, but some can.
In fact some people can be so good at hiding their depression, or pushing through it when they are with others that those around them just can't see anything, or that much wrong with them at all, despite them really struggling with intense feelings/effects of depression.
Now the DSM "should" be a good baseline for diagnosis but the very fact that it was this person who used it makes me wonder whether their interpretation has been off. And you know actually it should really be adequately trained professionals using it to diagnose which I'm betting this person wasn't!!!
So maybe worth going to someone else e.g. a doctor or pdoc??? or for now check out this link for an idea, doesn't replace seeing a professional but.............
Psych Central - Depression Screening Test
And perhaps tell us a bit more about what you're going through.........again we can't replace professionals but..............
Not that I'm necessarily saying you are depressed (maybe......maybe not.......) but in itself the fact that you're "sad a lot" says you maybe need some understanding, help/support with that, and towards working through that, towards feeling differently.
And whether someone on here is depressed or sad then it still matters, so keep on posting we're here for you
And have a think about seeing someone more qualified/a professional to diagnose if you have depression and to help you with that too??

Alison