http://www.msz.gov.pl/en/p/msz_en/travel_to_poland/visa/visa;jsessionid=8457C5C6CA72617BAD8818E8C67C5783.cmsap5p

The national D-type visa entitles the holder to:
- stay in the territory of Poland throughout the period of its validity;
- additionally move within the territory of other Schengen states for up to three months within a half-year period.

If you have a Schengen Health Insurance and all necessary documents, you can enter any Schengen states before the start of your study in Poland.

http://www.go-poland.pl/visa-application

“D” type – a long-term national visa issued for up to one year allowing travel around the Schengen area for up to 90 days in a 180 days period A visa can only be extended in exceptional situations. Should you need to stay in Poland longer than specified in your visa please do apply for the temporary residence permit (see below)