Translation:The school is tall.
I guess the /n/ sound in Korean is more sort of dental rather than palatal (as it is in English) So you should kind of pronounce ㄴ using your tweeth somehow (that's why most of the times ot resembles more a d rather than a n) Korean Unnie always say that each sound is unique in Korean and should be studied as such rather than comparing it to a sound of another language (at least this has helped me broaden my Korean listening skills (:)
It's not a dental, it's an alveolar: https://en.wikipedia.org/wiki/Korean_phonology#Consonants
ㄴ is pronounced more abruptly in Korean than English, which is why it can sound like a d sound. If you say the word "nope" slowly in English you'll notice that in English we always hum a bit before actually opening our mouths to make the n sound. This is not what ㄴ sounds like at the beginning of a word in Korean. Rather, it sounds like halfway between a n and d.
P.S. The fact that we tend to voice (hum) before saying a consonant in English is also why at the beginning of a word ㅁ, ㄷ, ㅂ, ㄱ, and ㅈ may sometimes sound like b, t, p, k, and ch respectively to a native English speaker. They're all pronounced abruptly ㅁ sounds like halfway between m and b, ㅈ sounds like halfway between j and ch, and ㄷ, ㅂ, ㄱ actually sound like the t in stop, the p in spot, and the k in ski.
높다 means high, but I don't think the English "high" and the Korean one have the same rule. We would probably just say "big" in English, even if it is tall. It could be strange because no school in America (that I know of) is taller than 2 floors, yet the schools in Korea can easily be 5 or 6 floors normally.