A good grasp of English is extremely important for succeeding in law - truly, the only two skills you need to succeed are diligence and reading comprehension. Writing ability can be learned.
It's not about how quickly you write an essay (particularly high school essays, which differ so much from law essays that they are incomparable), but how well you read; the depth and versatility of your vocabulary; and the fluency of your verbal skills (oral and written).
Law essays tend to test synthesis (research, adducing arguments) as much as they test analysis. I found that in high school I had never ever had to synthesise information beyond a basic level and this was troubling. Law exams test how well you write under pressure.
A good way to test yourself is to read a few of the easier law judgments - particularly,
Donoghue v Stevenson, UK Law Online (Donoghue v Stevenson)
Justis - ICLR Special Issue - Carlill v Carbolic Smoke Ball Co. (Carlill v Carbolic Smokeball)
The above two cases are two of the most important ever. More to the point, they are interesting, relevant, short, and relatively easy. See how you go with those - it's ok to be a bit frazzled at first but if you really can't see yourself reading hundreds of pages of that stuff per subject, law school is not for you
