My school calls my mom whenever I’m absent. How can I block their number from my mom’s Samsung?

When I was a sophomore, my cousin was a senior. We’d always hang out & I skipped school ONCE & my mom found out because the school called. I’m about to be a senior & they still do. However, i haven’t skipped since- only been absent cause I stayed home or had an appointment. These next 2 days I have early release. We’re only going to 2 classes for finals (tests). Today I went & the school called saying I was absent but how am I supposed to be present at other classes if we have to go to specific ones we’re assigned to test in? So here’s the thing. My mom isn’t on board with me being on birth control. She literally called me a slut saying that I was going to fuck anything that moves if I get on it. I have an appointment tomorrow after school at planned parenthood to get the iud because I don’t plan on conceiving anytime soon. I turn 18 in 4 months & I honestly feel so trapped & stripped of liberty having to get on bc on the low. I want to block the school’s # just for one day so that they won’t call & let my mom know I have early release or say that I didn’t go to school. She has a Samsung & idk how to block it