Leonardo, unfortunately, you need to change the whole essay because you wrote exactly about what the prompt told you not to write. The reviewer is not interest in your high school exploits or extra curricular activities. The university could not have been clearer about it since they said:
The statement of purpose is not meant to be a listing of accomplishments in high school or a record of your participation in school-related activities.
Since you did exactly that in this statement of purpose, you have failed to deliver the required information and as such, have just failed to gain consideration as a student at UT Austin. However, it is not too late to repair the damage. The damage is not permanent yet since you have yet to submit the document. Here is what I advise you to do in order to correct the paper:
1. Forget about high school and your extra curricular activities. This is not a college application essay. This is an essay that is asking you to tell the reviewer why they should consider you for the coveted slot of a transfer student. Therefore, your opening statement should be all about your interest in the field of Physics and how you have come to accomplish certain notable achievements in this field. This is all about who you are now, not who you were in high school. In fact, since you did not do well academically in middle and high school. it would be in your best interest to never mention it to the reviewer. The reviewer is not interested in the story of how you were a slacker in the 10th grade. They also do not care that you can play the violincello. Those stories and other activities are not related to your interest in Physics. Anything that can't circle back to your course at present and in the future should be left off the page.
2. Discuss the career plan that you have for your self. Since this is a statement of purpose, I can only assume that you are trying to transfer schools as a masters degree student. Therefore, you already know what your abilities are, how you developed those abilities, and where you hope advanced studies, combined with your previous experience and on the job knowledge, will lead you. That is one of the most important points that you have to make in a SOP. Without a career plan, you can't really convince the reviewer that you are going to complete the course. Advanced studies are not easy to complete so they need to know that you are dedicated to doing whatever it takes to see this action through to the end. Also, explain why your current school no longer suits your masters degree aspirations.
3. Expand upon your current research as it relates to your chosen field. Take note of and mention the ways that you feel UT Austin will be able to help you further advance that research. Be specific about the internships, training, or exchange programs they have which you plan to fully utilize in order to give you a better foothold in the world of Physics.
I strongly advise that you take note of my suggestions and write a totally new statement of purpose. The one you have right now should not be submitted. It won't accomplish the task it was expected to do with the reviewer. Take the chance to better address the requirements of the prompt. We will be here to assist you :-)