i think my friend is self harming

i have never seen her wear short sleeves, even in summer and that could just be because she’s insecure. she talks about being insecure about her weight (she not really under or overweight and she’s geougeoys( idk how to spell)) but she makes jokes about wanting to off herself and always says “i’m joking i’m joking” but is she? i’ve seen her put bandages in her pocket before asking to go to the bathroom, and i could be so wrong but i’ve been clean for a bit and this is exactly what i thought and did. what do i do, should i say something to her? a teacher?