From the website:
Assessors will take note of the circumstances under which the work was done, and assess it accordingly. A very different standard of content and presentation will be expected from pieces of highly prepared course work, on which many hours have been spent, than from pieces of written work for homework with a short dead-line, or written under examination conditions
Your essays will be judged in context, do not worry. The tutors will also consider your school and probably recognise that not much is offered for Oxbridge students if most people are getting Cs or below.
The fact that there are minor spelling and grammar mistakes is hardly the end of the world. On one hand it shows that you do not read through your essays before giving them in, but this is merely a matter of habit not ability. On the other hand it is at least a mark of some authenticity - that the essay is a real piece of homework. One of my essays for PPE (a history one) contained a few spelling mistakes and my teacher said I could not amend it. At first I was annoyed and thought that had I known I would be submitting the essay to Oxford I would have certainly read through it but I now see it is no bad thing. So many people bend the rules and rework essays to the extent that the essays themselves become of little value to admissions tutors.
Don't worry about the length or the sophistication of your essays either. The aforementioned History essay was a homework essay that I wrote in less than three hours and was 2,000 words. I was slightly worried when my teacher wrote on the coversheet that I had 3 weeks in which to complete the essay. Although the teacher did set out the term's work at the start of term I did have another History essay for the week before, so effectively had a week. The other piece I submitted was an Economics essay on the Euro which was a two part homework essay that I decided instead to combine as one and do some further research on which came to over 4,000 words.