Don't get me wrong, but Depression is no laughing matter, trust me. However it sounds like this particular depression (if you say what it is) is... life. That's how life is. I worked and did school at the same time too. It blows, but that's how it is.depression stems from trying to juggle school with having to maintain a job
If the teacher is singling you out, say something to him after class, or speak with another teacher about it. There are resources to deal with this, but you don't seem to have taken advantage of them, and now is a bit late in the game.
Yes, they can fail you, depending on the state. Your age does not matter.
You said you "Never failed a class" but that you also don't understand anything in your Spanish 3 class (Also implying that you took 1 and 2) which means you cant "not understand anything". It sounds like you've just slacked off for a semester in your spanish class and not bothered to get outside help where it was obviously needed.