pronunciation - Why are "look" and "school" pronounced differently?

I can't figure out a rule that says whether 'oo' makes one sound or the other. Same number of syllables: one closed, but the sound is different.

Answer

There are no 100% reliable spelling or pronunciation rules in English but there are exceptions and patterns which can be recognized and memorized.

My accent is British and the following words are how they are most often pronounced in the UK. American English may or may not share the same pronunciation.

The letter combination, oo, usually has two sounds: /uː/ and /ʊ/.
School, boot, and choose are pronounced as /skuːl/, /buːt/ and /ʧuːz/

Good, took, book, and look are pronounced: /gʊd/, /tʊk/, /bʊk/ and /lʊk/ but foot which logic dictates should imitate the long u vowel in boot is instead pronounced as /fʊt/. However, the short vowel sound is kept in football which is pronounced as /ˈfʊtbɔːl/.

Other words which are spelt with oo include blood and flood which are pronounced respectively /blʌd/ and /flʌd/ but food which could have the short vowel as in good, or share the same vowel sound as in flood is pronounced as /fuːd/!

In other words, you need to memorize the pronunciation for each English word and when in doubt, check with a dictionary (or ask a native speaker to help you out).