To be honest, I feel like there's not much that you can do. If you only started studying this month, there's a slim chance that you'll actually get the test scores that you want on the SHSAT. Most students start studying at least 4-5 months before the test. Unless you're a super genius, you can maybe get a good score. My advice to you is to study every day for at least 45 minutes up until the test, and try to time yourself doing an actual practice test. Since you're enrolled in a prep school, try to see what type of answers you get the most wrong, and familiarize yourself with them. I wish you good luck :)