I agree with the advice Peter has given you. Either 'secondary school' or 'high school' would be fine, and also universally understood. I'd avoid using the word 'gymnasium', as this is likely to be misunderstood as a fitness centre! I would also avoid using the term 'comprehensive school' as this can have negative associations for some 'education snobs' in the UK.

Thank you a lot. I had doubts about using the word gymnasium as well, I think I'll just put 'general-education secondary school'. I don't think the employer knows those nuances anyway, but I was curious...Thanks!

It depends on to whom you will be giving your CV and how well they are aware of the meaning of these terms. If you were intending to use your CV to apply for a job in an English speaking country, the term "secondary school" or "high school" would be well understood by most employers. In the English speaking world, the term "graduate" in reference to a person completing secondary school is mostly used in the United States and perhaps Canada. In other English speaking countries, a "graduate" is a person who has completed a higher education or university course. You could use the term "graduate' in reference to your completion of high school or simply say that you have completed your secondary education. The term "gymnasium" is certainly used in Germany to mean a type of secondary school as far as I know but you have to be careful of its use in the English speaking world where it will be taken, without further explanation, to mean a "gym" or fitness centre!