So, she is 189 cm and is in middle school? That would make her about 14 years or so. According to a wikipedia article on height by nation the average height of a Japanese female is about 156-158, so that would make Hiki over 30 cm higher than average and since she is still in puberty she is likely to continue to grow. I can understand her concern for her height, but when you are that tall at such a young age there are usually some medical condition causing it.