그 학교는 좋은가요? OR 그 학교가 좋은가요? Both can be used but the first one is more common. 학교는 sounds like a normal discourse centered around the subject of 그 학교. 학교가 sounds like asking or mentioning an incidental fact as part of another broader subject. The difference between 은/는 (topic marker) and 이/가 (subject marker) is a big subject. You can search for "italki.com: 은/는 이/가" for other answers about it, like this one: https://www.italki.com/question/405436

Well, I think that depends on the context. Both can be correct, depending on context. But I think it is most likely that 는 would be most relevant here. I recommend watching this video: https://www.youtube.com/watch?v=fCxLNRLntc0 But I think the sentence should be 그 학교는/가 좋아요? or 그 학교는/가 좋은 것 같아요?