though I have had people tell me they did work while waiting for the NIP.
They have to give you a NIP - they can't refuse one. It can take up to two weeks to wait for, but usually less. Depends on the workload in the town you're at. If a private language school say they can't pay you until you've got it (the oldest trick in the book) remind them that they can submit your tax form with a passport number instead.
You shouldn't need a work permit if you're from an English-speaking country. Opinions differ on this, and Polish-owned schools don't generally know about the latest regulations. Or pretend they don't so they can pop your wages in a term deposit for a month or two. One poor American guy I knew had to wait 5 months to get paid, due to Polish staff in the language school not wanting to make a potential wrong decision.
There's a Yahoogroup called Polang, somewhat defunct now but with a search function, also the ESL Cafe, both of which have information on the work permit issue. You still need a residence permit though.
Remember that in Poland, everything is down to the decisions of capricious minor civil servants, and things vary hugely from town to town, though Warsaw is slowly getting better. I've heard Krakow isn't.