In most words with a long U that doesn't start a syllable, it is pronounced /uː/. Examples: student, reduce, introduce. However, in some words (such as music, mule, human) it is pronounced /juː/. I've heard that in other countries, even the first examples are pronounced like the second (/ˈstjuːd(ə)nt/, /ɹɪˈdjuːs/, /ɪntrəˈdjuːs/).
What's with the discrepancies in the American pronunciation?