what docs can suffice as proof of intention to leave Poland after studies.
To prove intention, provide solid documents that prove that, in your home country, you have:
1) a well-paying, high-status job of at least several years duration;
2) a substantial amount of immobile property, like a comfortable house or sizable and profitable farm;
3) your own successful business, or a working interest in a corporation, of significant value;
4) a substantial amount of net savings and/or investments in your local economy; and
5) play a significant role in your local or regional community hold an significant office on a local, regional or national scale.
All of which show that you are thriving in your homeland and would lose a lot if you were not to return to your homeland. The more of the above you have and the stronger they are, the better. The fewer and the weaker they are, the more likely the consul is going to suspect that you plan to violate the terms of your visa and stay in the EU.
Naturally, for a student this is very difficult. You'll just have to provide what documents you can, and hope that the consul finds them convincing. For example, if you have held a job in your own country, it shows that you have been able to find employment there. If your family is wealthy and owns a family business which you will inherit, that is also convincing. But this all has to be on official paper.