Whether you need an exam in the preliminaries of high school math competitions depends on the regulations of your school or region. Generally speaking, the preliminaries of high school mathematics competitions are to select excellent contestants and provide them with opportunities to prepare for war, so most schools or regions will require students to participate in the preliminaries. If you are interested in taking part in the math competition in senior high school, I suggest you know the regulations of your school or region first and prepare carefully. Even if you don't have to take the exam in the preliminaries, you can actively improve your math level and competition experience by participating.