I'm sorry to hear you don't have a great relationship with your parents, because honestly, they should know. Regardless of any tension, keep in mind that they love you and want the best for you. Most teenagers go through rocky relationship periods with their parents, but opening up to them about how you're feeling might bring you closer.
If you absolutely can't talk to them, try to find a trusted adult. If you trust your school principal, that may be a good start. Most schools have counselors on staff, too. I don't know where you live or what the laws are in your area, but in most places, yes, they do have to inform your parents IF they feel you are a danger to yourself or others. If you aren't, and you're just seeking help and need someone to talk to, there shouldn't be a requirement for them to tell your parents.
Best wishes to you.