This is human nature and there is very little you can do to prevent this. If you are resolved to deal with any negativity that this disclosure may bring, you will be sufficiently prepared for it. This is the most important question to ask yourself. Think very carefully if your reasoning is to hurt or shock people. Often the person who gets hurt will be you.
Many gay people describe how important it is to first tell someone outside the family.
Make sure it's someone you trust and who you believe to be open minded and supportive. Be careful if you decide to confide in a teacher at school - they may be obliged to tell someone else what you have told them. Find out the school policy on confidentiality before you go ahead. If you have decided to tell your family it may be easier to talk to one parent before the other. You could then ask them for help to approach the other.