Ukrainian police said that a window was smashed and two plastic cans containing an unknown liquid and an ignited material were thrown into one of the school's rooms on Tuesday night, Radio Poland reported.
The Polish foreign ministry said: "No one was harmed in the incident and the relevant Ukrainian services have already taken steps to explain the circumstances around the incident."
The Ukraine state-school in Mostyska runs classes in the Polish language and programs for ethnic minorities. School vacation in Ukraine started on May 27.
Read alsoUkrainian, Polish experts to address controversy over historical monumentsThe school principal said: "There are no ethnically-based clashes here," adding that there had not been similar incidents in the school's 15-year history.
This is the latest in a string of attacks on Polish sites including monuments and diplomatic posts in the Ukraine.
Kyiv condemned the earlier incidents and suggested that Russia could be behind the attacks.