Of course, it's best to explain to dad at this moment. You can hide it from mom first, or you can make a guarantee to mom or dad with the results of other subjects and let them buy it for you. In the case of ensuring your academic performance, improve your painting level as soon as possible and guarantee it with excellent works. I have a good friend here, who has never been exposed to painting and calligraphy before. He studies science. It was only because of his hobbies that the university learned to draw and write his current works after school. He drew them himself.