A new application period starts always in the beginning of the year and ends in March. Admissions are nowadays handled by the Admission Services and all queries about the admission should be directed to them. Information can be found here:
https://www.helsinki.fi/en/studying/non-degree-programmes-for-teacher-qualifications

You can read more information about the STEP programme here:
https://guide.student.helsinki.fi/en/article/subject-teacher-education-programme-step


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STEP in the Faculty of Educational Sciences in the University of Helsinki is geared for both Finnish and international students. The programme gives official qualifications for teaching in basic and upper secondary education in Finland. The whole STEP programme is 60 ECTS and it is designed in such a way that it can be completed with full-time studying in one academic year. The academic year starts in the autumn (Aug./Sept.) and ends in May.