One of my girls is small for her age and has been slow to loose her teeth (she lost her front teeth this fall in 3rd grade!). She looks younger than her age and younger than her twin.
Both my girls skipped and started 1st as 5 yr olds with Fall Bdays (turning 6). They are twins and are in the same class.
I will say generally- different age siblings are preferred to be in separate classes/grades.
I would avoid labels of any kind. My siblings and I all had varying levels of success in public schools and one of us was perceived as the 'smart one'. Later, as adults we found out our IQ scores were all within 5 points of each other.
