I think the reason for the inner eight characters is that the teacher didn't teach every child carefully enough.
In fact, if you really practice according to the textbook, this will not happen.
I suggest you practice standing at one's feet.
Feet and heels are close together, toes are outward, and the knees of the two legs are close together, so that the thighs of the child can be measured together. It is necessary to tighten the thighs inward and straighten the knees, so that the feet can form capital letters as much as possible, and then stand against the wall facing the wall, so that the toes and heels are best against the wall.
Repeat this exercise several times a day, and you will be able to stand well. If you stand well, there will naturally be no more internal characters.