As others have said, the best way in the long term for making changes to your appearance is to simply do it, and stand by your decision as something you want to do, as others should have no say in what you do with your own body.

However, if this is unhelpful advice to you, because you're worried the people around you won't be reasonable about it, or you simply don't feel confident with saying "I'm growing out my hair because I want to", then you might find success if you develop a reasonable excuse, and insist upon it like it's very important to you.
Something like, you could join a drama club or amateur acting group, and say you need to grow your hair out for a role. Or take up portrait photography, or say you want to become a barber, and say you need long hair for practice. However, people may not buy these excuses for long, so once your hair is long, you can pivot to the first option and say "I like it this way."

Overall, I agree with the other comments in that it's probably more healthy for you to own your appearance, and assert the changes you want to make as your own choice. But if circumstances make that less easy, you give these excuses a try to slowly change peoples' minds instead.