Circle time can be extra challenging for children who have a hard time in school. Sitting on the floor with backs unsupported is very difficult for low tone kids.
 

There should be a variety of sitting options for circle time. Since no one wants to be singled out, I suggest that there be a few chairs around the perimeter of the circle at the beginning of the school year. The teacher can invite the children to try both sitting in them and sitting on the floor, and then deciding which they prefer. Eventually, the children who need them will go on using the chairs, and the rest of the children will choose the floor, and no one will notice or care who sits where because they have all tried all of the options and made their choices. If this is truly not possible, then having the child sit with his back against a solid surface, like the wall, is the best choice. Or perhaps a few floor chairs could be kept in a cubby and made available.