My kids (Montessori) school has horizontal classes (3 age groups in one class) for exactly this reason. From ages 3-12 they go through the cycle of being younge

My kids (Montessori) school has horizontal classes (3 age groups in one class) for exactly this reason. From ages 3-12 they go through the cycle of being youngest, middle and oldest 3 times, teaching them valuable 'soft skills' such as you described.