Hey there, I know it is a scary thing people finding out about your problems but if you speak to them about it will help you deal with your cutting. Your friends though they may not be doing the right thing by giving you an ultimatum, they are looking out for you and are scared for your welfare. They don't want to see you seriously ill in hospital. That is not what you want, right? Your friends have your best interests at heart. Perhaps you should say to them that you need a bit of time because cutting is not an easy thing to give up. If you say to them that you will see the guidance people of you own free will because they cant force you into something you don't want to do.
They want to help you, so let them help you. It might be easier if you have a group of people around you, supporting you either way it is going to be hard. I think the school has the right to pass on information like that right? Because I'm not sure. The guidance people will be able to help, you. If you show your friends that your not going to cut then they should not tell the teacher. So if you can don't cut because that way they have a reason to tell the teacher, use that as an incentive. It will be easier if you go through this with help, not on your own. But you have to be ready to give up, because there is no point if you don't want to give up yourself. When you feel down, do something else to keep yourself occupied, that way you wont be as drawn to cutting as you normally would. I know it is not always easy talking about your problems and seeking help, but believe me it is the best thing to do.
Good luck, PM if you want to at any time, ok :)