Which is correct? Elle a des cheveux longs ou Elle a des longs cheveux?

You're right, long is listed in the lesson as an adjective that usually precedes the noun. The placement of adjectives is nothing that's written in stone, however. There are two versions in common use:

Elle a les cheveux longs. -- This version is the accepted standard phrase in everyday French.
Elle a de longs cheveux. -- This is also correct but not used as frequently. It sounds a bit higher register.

Which Kwiziq question or lesson please?

Elle a les cheveux longs. Elle a les cheveux courts. This is what you'd use if you're describing a person. It's going to use "les" with cheveux or yeux. Also, you're going to put adjectives like longs/courts/blonds/roux behind the word cheveux or yeux (and make sure to use masculine, plural adjectives).  Was this in reference to a particular lesson, though? I'd always write these the way I did previously, unless there's a specific context I'm missing.

Thank you for your response.

This is with reference this this lesson - Position of French Adjectives - Short and common adjectives that go BEFORE nouns - which teaches some adjectives that must go before nouns. So adjectives like - long/longue, court/courte, petit/petite - basically BAGS adjectives that describe - (Beauty, Age, Goodness, Size / BAGS). 

So it is "Elle a les cheveux longs" and not "Elle a les longs cheveux". 

Similarly, it is "Elle a les cheveux courts" and not "Elle a les courts cheveux".

So these lesson rules don't apply here? 

What about ... "J'ai les cheveux laids"? Or "Elle est une laide fille"?