I would say "Do you know of a good school?" or "Do you know of any good schools?"
The first one might be correct, but the second one is in plural and has a different meaning (compare wanting one example or multiple examples)
The first one is correct.
What's the difference between "ismerz" and "tudz"?
Tudni is to know how to do something. Ismerni is to know (ie be aware of) a person or thing.
Igen, Hogwarts jó iskola