I was taught that THE was a definite article and under certain
circumstances an adverb e.g. "the more the merrier". 
My son's teacher insists that THE is an adjective.

Sounds wrong to me, but I'm a relic who was born in the 1940s. ;-)

Apparently things have changed. Here are some mentions of "the" as an adjective:

"while the is an adjective, it is also classified as an article, and
specifically as the definite article."

http://cleanfun.cz/efl/sgcthesp.htm

"Teaching students the concept of modification is crucial, and not
very difficult -- if instruction begins with the students previous
competence with the language. The students, for example, could be
given the following sentence and be asked to break it into three
pieces.

The young children slowly walked across the lawn.

Is any child going to break this into: 

The young   /   children slowly   /   walked across the lawn.

No, their understanding of English will probably result in all of them
breaking the sentences as:

The young children   /  slowly walked   /  across the lawn.

The concept of modification can now be explained by showing the
students that the words that ('go with,' 'describe,' 'chunk to,' or
'modify') nouns, such as 'children,' we call adjectives. Words that go
to verbs, like 'slowly,' we call adverbs. The primary reason for
teaching students to identify adjectives and adverbs is simply to give
them terms with which to look at and explain syntactic connections.
Note that with these terms, fourth graders can explain how all the
words in our sample sentence are connected. 'The' and 'young' function
as adjectives to 'children'; 'slowly' is an adverb to 'walked'; 'the'
is an adjective to 'lawn' within the prepositional phrase 'across the
lawn.' The prepositional phrase, in turn, is an adverb to 'walked,'
and the subject 'children' goes with the verb 'walked.' "

http://nweb.pct.edu/homepage/staff/evavra/kiss/RPP/GC_04.htm

Articles are adjectives.

-- mathtalk-ga

Yes, articles are adjectives, but the definite article is a special
type of adjective, and fuddy-duddies like me hate to see it lose its
"specialness."

Pronouns are nouns. Have today's teachers dropped the concept of
"pronoun" from their lexicon? I hope not.

Limiting adjectives:
http://www.ucalgary.ca/UofC/eduweb/grammar/course/speech/1_4a.htm

An interesting article here:\
http://webster.commnet.edu/grammar/determiners/determiners.htm
discusses the use of the word 'the' as a determiner, a special kind of
adjective that is always followed by a noun.  All articles can be
considered adjectives because they always describe nouns.  So in this
sense an article is always an adjective, albeit a special kind of one.

Teachers are not always right.

On one occasion, (possibly when she was 8 or 9) my elder daughter had
used 'sentence' in an essay.

Her ENGLISH teacher marked this as a spelling mistake and told her to
write out 'sentance' ten times.

When challenged, the teacher admitted that her own spelling did leave
something to be desired.

And this was at a very expensive school that prided itself for its
academic standards.

Ah. Bryan's story reminds me of my 3rd grade teacher. I used to
correct her spelling lists. My favorite to this day is "Poison Ivory."

Sincerely,
Boquinha-ga