First of all....we don't really keep track of "grade levels" in our Home-School. Second of all...older kids can glean what they can from younger level work and visa versa.
Galilee and Jemima are starting an Anatomy/Physiology Course in a few weeks. It will be taught by a Master's College Prof. Until then...they are joining in on the youngers Apologia Astronomy Class.
Maybe they will stay with it....even after their olders class begins??? I think so.
Today in History.....we discussed Eric the Red and Leif the Lucky. The older girls will need to delve a bit deeper. For Zion...an introduction is adequate. Same topic...varying ages/grade levels. The same Time-Line for all will be assembled and hung on the wall.
While the older kids work on their Teaching Textbooks Algebra 2 lesson in the front yard....Zion works on her Saxon Grade 3 lesson in the living room.
I oversee things. It's my job to make my kids correct their work until they master the concepts presented.
That's how it's done.