Depends on what it is exactly. Need further info from you to be more precise.

Well, yes, I know, I'm supposed to put an actual translation proposal, one and only one, in the box above. And yes, I know, you want first-hand people from lakeside dwellers.

But if the following is to be believed, it is basically the French lycée général, as opposed to the lycée professionnel, only starting/finishing a year later.
I don't know about you, but I call lycée (unqualified) as "high school" as opposed to "middle school" (collège).
Of course the Swiss have this distinction between gymnase and école de degré diplôme (but they probably don't have classes préparatoires having, from what I can make out, set the bar earlier).

Secondary education, which is also mandatory, is for children ages 12 to 15. The curriculum has a general core, but offers classes to prepare for further optional education based on a student’s interests. Students may prepare to attend the “gymnase” or “GRAMMAR SCHOOL” (HIGH SCHOOL), a vocational school, or a general high school.

Optional continuing education, where students may pursue one of three types of studies.
• A “gymnase” leads to a “Maturité Fédérale” and subsequent entrance into a university,
• A vocational school (“école professionnelle”) or apprenticeship, which follows three to four years of company-based training and leads to a diploma for specialised studies (CFC), or
• A general high school (“école de degré diplôme”) leads to a “Certificat de fin d’études”, which is a high school equivalency diploma.
http://members.impactgrouphr.com/reloads4.nsf/73ceb0b9d43d52...

"Grammar school" might still be understood by many. Otherwise something "explanatory" like "gymnase prematriculation high school".

You might also do worse than the same thing in German, Gymnasium which might well be more readily understood than its French translation.

• Switzerland (usually 3–4 years after 9 years of compulsory schooling (primary and secondary I); in some cantons it is also possible to attend a so-called "Langzeitgymnasium" which lasts 6 years, following a six-year primary schooling; the Gymnasium ends with Matura at the age of 18/19).
http://en.wikipedia.org/wiki/Gymnasium_(school)

From what I understand of the Swiss education system, a Gymnasium is a secondary school for 11-18 year-olds, so Sixth Form College is definitely not the correct term. UK parents would understand what was meant by either academic or selective secondary school. Alternatively, you could follow Bourth's suggestion and keep the original term Gymnasium but provide a gloss.