The spelling system of English was never phonological, but entirely historical. Native speakers of phonetically orthographic languages such as Spanish often find the idea of learning to spell amusing. Learners of English are usually dumbfounded as to why we spell words like "rough, plough, through, and though" the same, but pronounce them differently. The many inconsistencies between spelling and pronunciation result from regional dialects in England developing independently prior to the inventing of the printing press, which froze our spelling habits in its current state rather than alleviating the differences. Words in Old and Middle English were spelt differently from region to region, and even by the same authors in the same works!
So basically, tell your teacher to go shove it when they circle spelling mistakes. There is no phonetical precedence for spelling 'correctly' in English other than for helping foreigners learn the language. Our phonetics are so ****ed up that 'proper' spelling should be considered trivial. Back in 11-dickety-something, if you were from Northumbria, and read something written in London english, you'd probably have no idea how to read it. Considering that, spell your English however you please.