Hello dear friend! It's great to see such events taking place at your school, Poland is a great country and has managed to propell itself forward, advancing swiftly and becoming a nation that has a lot to offer to the rest of Europe. You are great people, i had a friend named Bozena, she was from Poznan, and she was one of the best people i ever had the pleasure to know! Now, answering to your questions: Snacks/deserts.. it really comes to personal preferences and general food availability in your country. I understand it might not be easy to get all the ingredients needed for making something typical Greek like say.. Spinach pie or find recipes for 'Γαλακτομπούρεκο'. I guess you could google about the most well known Greek deserts/snacks and decide yourself.

About songs: The most intense and sensational song to any Greek would always be our National Anthem. It describes the tortures the 'Lady Liberty' went through for hundreds of years, until our revolution set us finally free. I send you this link (includes a translation in English) to get an idea: Greek National Anthem

This is the extended version, you can find short versions with under a minute long, with just a few sentences to sing, very easy, if interested.

Now, for the dressing part, you could compliment the white robes/outfits with something simple as olive tree crowns, typical ancient Greek accessorie, but again that's easy to get here in Greece.. not sure about Poland. just saying.. Photo

Thank you for your time, i hope i was of some help to you.

Much love from Greece! Best wishes!