Logic of English teaches the phonogram wor, as in worm, to explain the spelling of /wer/ like in work, worth, and worship. The pronunciation of words with the phonogram wor is almost always /wer/.

However, a few words like warm, war, and quarter have an unexpected spelling. The spelling does not match the pronunciation of /or/. When words have an irregular spelling, it is helpful to look at their spelling patterns. 

• this is an odd, but consistent spelling pattern
• after the /w/ pronunciation, the vowel in these words is pronounced /or/
• it occurs in a relatively small number of words
• some dialects pronounce it closer to /är/

Words with this interesting spelling pattern should be taught through spelling analysis. For example, you might say, "We usually pronounce this word /worm/, but we will say-to-spell /w-AR-m/ to help us remember how it is spelled." This is the same process we use with words containing a schwa sound or any other sound that is distorted or obscured in common pronunciation.