Mittelschule is a German term literally translating to "Middle School" (i.e. a level "intermediate" between elementary and higher education). It is used in various senses in the education systems of the various parts of German-speaking Europe, not necessarily equivalent the English term middle school (which is itself used in various meanings in various parts of the English-speaking world).
Examples of such use include:
Austria, lower secondary education for pupils aged 10–14, see education in Austria
Germany
in some States of Germany, a school analogous to a Hauptschule
in other States of Germany, a combination of Hauptschule and Realschule
an education level positioned between Hauptschule and Gymnasium (i.e. the middle group of pupils in terms of academic ability in a system of parallel schools).
Switzerland, see also education in Switzerland
a general German term for a Gymnasium, a secondary education, either for ages 15–18/19 (Sekundarstufe II, upper secondary education) also known as Kurzzeitmittelschule a.k.a. Kurzgymnasium, or then for ages from 12-18/19 (Sekundarstufe I & II, lower and upper secondary education) also known as Langzeitmittelschule a.k.a. Langgymnasium, both leading to university access with a Swiss Federal Matura graduation. There are, however, slight differences among cantons.
Other terms used in Switzerland are:
German: Gymnasium, Kantonsschule (cantonal gymnasium managed by the canton; or short: Kanti, or KS), Kollegium, Lyceum, Maturitätsschule
French: Gymnase, Collège, Lycée, Gymnase/Lycée Cantonal (cantonal gymnasium managed by the canton), Lycée-Collège, Lycee-Colege, Écoles du secondaire II, École publique Supérieures
Italian: Liceo, Liceo Cantonale (cantonal gymnasium managed by the canton)
Romansh: Academia
Fachmittelschule (formerly Diplommittelschule), secondary education (Sekundarstufe II) with a Fachmatura graduation leading to access to Fachhochschule tertiary education, but not to university access.
