If you are not a citizen of one of EU, EFTA or EEA countries, then yes, you definitely should legalize your stay in Poland. First of all, you have to find out whether you are allowed to enter the territory of Poland without a visa or not. List of visa-free countries is available on the website of Polish Ministry of Foreign Affairs. If your home country is not on the list, then you should apply for a visa type ‘D’ (Individual Country Visa) with ‘09’ sticker (pursuit of first cycle, second cycle degree, uniform master’s degree or attending a third cycle studies programme) in Polish consulate (search engine for Polish offices abroad can be found here).
Students who have entered Poland on basis of visa-free movement and those whose visas are about to expire have to legalize their stay. You can read more about procedures of legalization of stay here.
