The school are in loco parentis of you while you are there, which means they have a duty of care to protect you and this means that parents will generally be informed. But schools will have a lot of experience of dealing with this, and they're often very good at dealing with it calmly and giving out information but treating pupils with as much confidentiality as they can.
How a teacher reacts to it will differ, but they can be great. Not all teachers will generally be informed, but if necessary they can be (e.g. our asst. head let all of my teachers know so that if I had a very bad lesson they would speak to him). They're unlikely to see you any differently besides that they will know you are struggling, so they can be there to help if you need it, or to refer you to people who can help, such as school counsellors.