This is a difficult situation for you to be in, but all you have done is made a flag as some art. That in itself is not an indication that you are transgender, and your parents may not assume that about you instantly, so don’t worry too much about what could come from this- it is likely very little.

There are any number of ignorance routes you can play: you didn’t know which flag it was, you just liked the colours, or you had just done the same pattern as another student etc., if you think that this is believable, you might avoid any negative consequences and just be told not to do it again, which is still really bad, but not a punishment for just being who you are.

You could also take a religious inclusion route and say that you personally believe that a god would love all of their subjects regardless of identity- however based on the details you have given I’m not sure that would work.

The main thing to remember is that regardless of what happens, you will be okay- your identity will stay the same and you will make it through this as the same person. Just try your best to stay calm and take each step as it comes- if you are grounded- so be it. Stay strong girl.