If you notice when the kids start at thier new school Loraine and Charlie are in high school and go together. Sarah, Jake, Jessica, Kimberly, and Mark go to middle school. Henry, who is in that mix, doesn't go to school at all i guess?
Actually the kids that are in the groups are as followed:
High School: Charlie (12th) and Loraine (9th)
Middle School: Henry (7th) and Sarah (6th)
Elementary School: Jake (5th), Mark (4th), Jessica and Kim (2nd), and Mike (1st)
Kindergarten: Kyle and Nigel
The Producers just didn't want to spend time seeing Henry and Sarah off at school because they didn't have any problems with the family other than the move.