Spelling consistency is consistency of spelling. It's often expected in written texts, including student-written texts. That is, one should consistently employ a spelling system, e.g. American spellings or British spellings, but not mix them. Additionally, if one uses a word with various alternative spellings, e.g. "judgement" and "judgment", one should be consistent in using a single spelling for that word.
Finally, using American spellings with British grammar, or British spellings with American grammar, can also seem strange.