Presenting 324 words may prove to be a problem for you during the actual test that only allows for 40 minutes of writing, proofreading, and final version creation. Based on the amount of time required to ensure that you submit a grammatically perfected essay, you should not be writing more than 300 words. In fact, the essay has several problem points that you were unable to spot because you focused on length rather than quality.
For starters, the thesis statement is missing in the first paragraph. You need to provide a summary of the upcoming discussion, by offering the upcoming topics for discussion to clearly show how well you understood the prompt and the requirements. You do not have to discuss those subjects in that paragraph, but you should not waste the TA score by simply repeating the writing instructions either.
The causes paragraph is clear at the start, but then becomes confusing to read when it gets to the second discussion topic. Instead of saying:
Also, a busy life takes responsibility for a poor diet, as well as the second main reason.
Go directly to the point and indicate that this is the second reason. Clarity in the sentence is required to help keep the cohesive and coherent presentation of the paragraph. One mistake in the sentence formatting results in confusion for the reader. Which is one of the reasons why you have to focus on quality rather than quantity when writing these essays.