To apply for a Polish visa, you must first be accepted by an educational institution in Poland for admission. After you are accepted, they’ll send you an enrollment letter which is required while applying for a visa.

Documents required to apply for a visa:
Completed application form
Two recently taken passport-sized photographs
Valid passport with at least six months of extended validity
Enrollment letter
Previous transcripts authorized by External Affairs office
Proof of payment of admission, examination, and first year semester fee
Proof of sufficient fund and income
TOEFL or IELTS score card
Health Insurance certificate

If required, you may be asked for additional documents.

All detail information on requirements and process to apply for Polish visa from Pakistan is available here:
http://www.islamabad.polemb.net/index.php?document=41