My children and I look at adjectives a little differently. I think articles, such as-a-the-an-, are not adjectives. But it is true that some people think they are adjectives, while others think they are articles.
Oh well, the English language is always open to interpretation!

“other languages have no adjectives at all. In English the set of adjectives is fairly well understood, though some people include other parts of speech — such as articles like the — in the class of adjectives.”-wisegeek

When making Montessori grammar cards, I would make articles(-a-an-the-) highlighted in gray, not blue-
As always, your students can decide what they think is correct.