A tricky one!
Possibly "qualifications" refers to vocational training such as a BTS and "diplôme" refers to "academic" diplomas, mainly the Bac.
For example, this extract from the Sénat website:
"Ce dispositif vise les jeunes de 16 à 25 ans, sans diplôme, sans qualification professionnelle ou avec un niveau de qualification inférieur à un diplôme de second cycle long de l'enseignement général, technologique ou professionnel."

In the UK at least, I don't think this distinction would be made. One would say "unqualified school-leavers".

Hope this helps a little.

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Note added at 7 hrs (2006-07-20 16:43:51 GMT)
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Everything I've looked at online refers to "unqualified school-leavers". "Unskilled" is used to describe a job that doesn't require qualifications. So if you are unqualified you're more than likely to end up in an unskilled job. I've been trying to find out more about this "international definition" your text refers to but no luck so far. If you have any more details of that it would be something to follow up.
One way to make the distinction could be "no vocational or academic qualifications."

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Note added at 7 hrs (2006-07-20 16:46:08 GMT)
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From a UK government website

Choosing between academic and vocational qualifications

From GCSEs to GNVQs there are lots of different qualifications to choose from. If you're thinking about learning as an adult for the first time you should decide whether to study towards a standard school qualification or something related to your work.
The benefits of academic qualifications

An academic course could be right for you if you want a qualification that shows you've achieved a certain educational standard. Traditional academic qualifications you can get from school demonstrate a certain level of analytical and critical skill. These are important skills needed in most jobs.

Many jobs ask for a good standard of education, such as five GCSEs (General Certificate of Secondary Education), good A levels, or sometimes a degree. Often they won't specify particular subjects, they just want evidence of general educational achievement.


The benefits of vocational and work-related qualifications

For some jobs and professions you will need to demonstrate practical skills as well as theoretical knowledge. Some employers value a practical vocational course that provides the specific skills needed for the job. You might prefer a vocational qualification if you know what job you want and you enjoy learning practical skills.


This works if we can be sure that this is the distinction being made in French!

You might want to post this in the French-French section.
My understanding of qualification is that it is some achievement but a lesser achievement than a diploma/degree.

For example: Bob finished 2 years of medical school. He is not degreed but everyone knows that he has some knowledge.

Also there are some "étapes" for French adolescents that may be required for continuation but do not merit a diploma.

Failure: a student who leaves the school system with no specified level of achievement nor a diploma.

To certify individual students as having attained specified levels of achievement. darwin.nap.edu/books/0309083001/html/30.html