I think talking to a therapist would be the best choice. Of course it may be more of a challenge depending on if you are old enough to just find a therapist on your own or if you have to get your parents to do it. Another option would be a school counselor. You can also talk about it with your regular doctor. Be sure to make it clear whether you are feeling suicidal or not, and just be honest about the self harm.

I also self harmed in many different ways all through my childhood and teens. I finally told my therapist about it and then my parents and friends. During my teens I often spent a lot of time lost in my own head envisioning self harming and envisioning harming others. I cant even remember if or how much I ever talked with my therapist about the harming others thoughts. But I'm going to be bringing it up at my next therapy session because those thoughts are creeping back, I may need more medication adjustments or therapy more often.

After my early 20s I went years without self harming. Only recently after several traumatic events have I started to self harm again, and I'm 38 now.

I hope you can find some help from someone you trust, and don't give up if the first attempt to reach out for help fails just keep trying. Sometimes it's easier to text or Wright someone about it rather than talking face to face.