Since you said it's a Moldovan certificate, I tried translating the phrase literally into Russian to see what would happen, and sure enough there is a dictionary entry in Multitran for "неполная средняя школа" (неполная = incomplete, средняя = middle, школа = school):

(амер.) неполная средняя школа junior high school (7, 8, 9 классы)

(образ.) неполная средняя школа junior secondary school (которую дети посещают с 11-12 до 14-15 лет)
http://www.multitran.ru/c/m.exe?a=phr&s=�������� �����&l1=2&l2=1

The above explanations say that children begin this type of schooling at the age of 11-12 and end it at the age of 14-15 (known in Russia as the 7th, 8th and 9th classes). Obviously this English translation would be more appropriate for US English rather than British (for which "middle/secondary" might be a better bet than "high").

This is as opposed to a полная средняя школа (literally "full middle school"), for which it suggests "senior high school". It says this is for children in the 10th-12th classes:

http://www.multitran.ru/c/m.exe?t=798027_1_2

The same translations are given here (from US English into Russian):
http://cito-web.yspu.yar.ru/link1/metod/met9/node28.html

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Note added at 5 hrs (2008-04-24 07:29:29 GMT)
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And when I say "classes" above, of course these would be "grades" in the US.

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Note added at 6 hrs (2008-04-24 08:14:16 GMT)
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Also found this:

"Secondary school
Secondary education in Russia takes either ten (skipping the 4th form) or eleven years to complete, depending on the school. After graduation from the 9th grade, which is compulsory for all Russian citizens, a pupil obtains a **Certificate of Incomplete Secondary Education**. After that a pupil has to choose one of the following ways to complete his secondary education..."
http://en.wikipedia.org/wiki/Education_in_Russia

Whether it's better to use "junior" or "incomplete" would probably depend on who will be reading the translation; but if you're using "incomplete", I think an explanation/footnote would help the reader since otherwise they might assume the student did not go on to complete the rest of his/her secondary education (which may or may not have been the case).