Threatened🔪

What do I do if someone threatens me.. This girl (let's call her Alex) threatened me saying she's gonna stab me, and this isn't the first time she's said it either, the bad thing is tho that she threatened me in front of teachers and they haven't done anything about it, and I'm kind of scared to go into school on Monday in case she actually did stab me? Should I get the police involved or just wait and see if she ever did do this to me