Hi Mari! Here are the changes I recommend to make your writing more natural. There are a lot of changes but I hope you can see what I was trying to do. Feel free to ask any questions about the changes and thank you!
Today I agreed to go to an English academy with my mom to take the level test. The test was supposed to be an hour long, but I finished quickly. This was because there were so many questions I didn't know the answer to so I didn't have to spend much time on each question.
Soon after, the teacher came out with my graded test. My mom looked at my score and frowned. It was okay because I knew what my score would be.
My mom poked me on my way home.
"How could you get that score? I was embarrassed."
I said to her, "what do you mean? I decided to go to the academy because I’m bad at English. If I was good, why would I go to the academy?"
And my mom replied, "then keep going until you can talk back like that in English!"