When you say that a school is "nice", you mean that it's a pleasant school to attend. When you say that a school is "bad", on the other hand, you mean that the standard of teaching is not high. So "bad" wouldn't be a good choice, in your example.
Here are some antonyms of "nice": disagreeable, horrible, nasty, repulsive, unpleasant.
See if you can guess which one is best, for your sentence. Then I'll check your answer for you.
As for the second question, both versions are possible:
1. High tides flooded many areas.
2. High tides have flooded many areas.
One is not more correct than the other: the correctness depends on the context!
Have a good week,