I remember doing an exercise with kids in my classes where we identified what type
of spelling errors they made eg wrong usage of the word eg there/their/they're missing out letters through carelessness, not knowing a spelling rule eg doubling the consonant before adding a 'y', missing a letter off the end, changing the tense part way through the sentence etc. Sorry, I can't remember all of them, but you get the basic idea. This helped to then see if there was a particular type of reason for their spelling errors so they then knew what to focus on, although of course it could be more than one type!
Going back into the classroom myself on Tuesday after 15
years absence so had better go and prepare!!