No, but it seems unfair to punish a kid with poor grades in ALL subjects for the crime of being poor in ONE.
If you're a sub-average speller, you should get subtracted points and thus potentially a lower grade in physics, math, history, biology, *everything* ?
There's a subject where students are graded for their english-skills already: it's called "english".
If the answer is not understandable, then obviously points are subtracted, but even then you don't subtract for poor english, you subtract for the fact that the student did not succeed in demonstrating knowledge of the subject area. (math, biology, chemistry or whatever)