There are several types of high schools in the Czech Republic. We place our exchange students mostly into so called "Gymnáziums" which are designed to prepare students for further education on universities.
On the Czech high school you will be part of a group of about 25-30 students that will be spending every day of the school week (Monday through Friday) with you. The core of the high school studies is to attend obligatory subjects such as Maths, Literature (and Czech Grammar), Physics, Languages (mainly English, German, Spanish, French, Russian) etc. Lately, many schools offer an interesting choice of facultative subjects. Attending a Czech high school will be a bit of a challenge but it will also leave you with tons of unforgettable memories. Being an exchange student on a Czech high school is unusual so be prepared to be a target of interest of your schoolmates at the beginning. If you open yourself to these new people, soon you will find good friends that will be more than willing to help you throughout your school path.