1 week is not much time...
I think you may be falling into a trap if you want to use "sophisticated" language and relying on a Thesaurus. You can often tell if someone has been using a Thesaurus to write "sophisticated" English -- it's difficult to understand.
Words like "it", "to", "that" and "is" are not only perfectly acceptable, they are absolutely necessary. These are words which have a grammatical function, so of course they're going to appear very frequently.
Language is not meant for showing people what big words you know; language is about communication, and the most important thing is that you communicate your ideas effectively. That means, among other things, not using complicated or obscure words where simple words would do.
Of course you need to demonstrate that you can use more "sophisticated" language and a variety of sentence structure in an exam, but you shouldn't overdo it.
A popular British sitcom of the 1980s which parodied the workings of the British government, Yes, Minister, made this point regularly. Sir Humphrey is a civil servant and is always using overly sophisticated language which the minister, Jim Hacker, could never understand. For example:
Sir Humphrey: "The identity of this official whose alleged responsibility for this hypothetical oversight has been the subject of recent speculation is not shrouded in quite such imprenetrable obscurity as certain previous disclosures may have led you to assume, and, in fact, not to put too fine a point on it, the individual in question was, it may surprise you to learn, the one to whom your present interlocutor is in the habit of addressing by means of the perpendicular pronoun."
Jim: "I beg your pardon?"
Sir Humphrey: (anguished pause) "It was I."
Looking at what you wrote, I'd say your biggest problem is actually the dreaded "run-on sentence". You write several main clauses and separate them with commas, which is very bad writing style. You should try to join some of the sentences up, but don't be afraid to shorten some of your sentences as well. You should aim for a variety in sentence length -- some short, some longer (but not too long).
For example, you write:
School is out and i still don't think i'm ready, i have 1 week, i want to write a good essay!
Hmm. That's four clauses strung together with just one coordinating conjunction. How about...
School is out and, with only one week left, I still don't think I'm ready. I want to write a good essay!
A very simple improvement -- and now it's easier to see the relationship between the four thoughts. And that's sophisiticated enough.
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