Silly question but I don't speak Polish.... Would that be a problem
Yes, a big problem as a schoolteacher.
I assume not as I feel the English spoken to me was better in Poland then In London
Remember, the people you spoke to would be people in hotels, restaurants etc who need it for their job in such a touristy place as Kraków and anyone you met socially would be speaking with you because they can - most people in Kraków (especially outside the tourist bits) do not speak fluent (or any) English.
And as Roz has said, the salaries are low - you would be shocked how low. Many teachers in Poland have additional sources of income outside their main job.