Foreign students can use the student discount on the conditions that:
1. They are citizens of one of the EU member countries.
2. They have an International Student Identity Card – ISIC.
3. They have a card confirming the right to a permanent stay in Poland.
4. They are under 26 years old.
All those conditions must be met.
Unless said student has a student ID issued by a Polish institution. Then said student will only need some other kind of ID and will be able to use said discount. But the age restriction is still in effect.