I am a bit older than David, so many of the things that you were taught in grade school were taught to me as well. I was also taught that articles were adjectives.
The public schools where I grew up were a few decades behind the times. When I was in high school, they were teaching that there were only two Kingdoms of Life, Plantae and Animalia, and that fungi and bacteria were Plantae, even though the scientific community had long since discarded that notion. The schools were equally lagging when it came to modern linguistic thinking.
David and I have had many discussions about such things, and we have come to agree that the traditional "parts of speech", as defined in most dictionaries, are inadequate. I hope to work with David on redefining some of these antiquated concepts.
Please don't be embarrassed about any misconceptions you may have. The language is still evolving, and David, Gustavo, and I are still learning it. And it will change tomorrow.
The whole broad category of "adjective" doesn't address why we can say "she wore a pretty blue skirt", but we can't say "she wore a blue pretty skirt", does it? Obviously, "a", "pretty", and "blue" don't follow the same rules.
May I ask where you are from? I would have guessed that you were a native speaker of English.