look at wiki 'education in England' .seconda superiore should be year 12 (of education)at college ,age 16 appr.Equivalent to High School but in Britain people use the term college (as they do the GCSEs)

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Note added at 6 hrs (2011-01-22 23:15:31 GMT)
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Dear Beatriz,
Your issue is a classical case of equivalence in translation and has been dealt with several times in translation practice. The best way to tackle the problem is by applying the theory of skopos of Hans Vermeer, as described by Munday (2002) according to which the translation to be chosen is determined by
1.the target readers' characteristics and
2.the function the translation is expected to have (Munday, 2002, p.40-60).
If the target reader is the British lay reader then you should opt for yr12 /first yr at college (or equivalent solutions), as this translation will be easily UNDERSTOOD by the target readers who are not expected to have background knowledge on a different system.
If ,however, your work is addressed to English-speaking Europeans (i.e.through an international school advertising brochure) then Julie’s ‘High school’ option might be a more suitable solution as the reader will not need to decode the unclear message through the British system curriculum (which they probably do not understand), but simply use the English language as a means to compare the Italian educational system to their own educational system , provided it is similar.
However, remember that English talk about school (junior and secondary) and college. That's all. The Wikipedia link on British education shows clearly the comparison with other curricula. It is part of your job to decide what to use and that will show your skills in translation!Good luck with your assignment,
Bolb123
Munday, J.(2002).Introducing Translation Studies.Theories and Applications. New York: Routledge