How is my position named? My advisor is responsible for assigning teachers to conducting excercises to lectures. Since there is a lot of students (and groups) sometimes last year students help by taking excercises in some groups. For the past 5 months I've been employed by the university and I've worked a couple of hours a week taking classes, without a formal position at the university (and for sure won't have until I finish my masters). 

Right now I am writing a CV and I would like to include the last 5 months in my work experience but I don't know how to name my position - is "Math teacher @ university" a good one?
Having worked at a university in HR, you are classified as an employee and you definitely have a "job title". Since you're a graduate student, you could be classified as a "Teaching Assistant" or "Graduate Assistant". I would recommend reaching out to your HR department and simply asking them what you your title is in the system.