In teaching school, students should start TAing, eventually teaching freshmen in their final years.
When they graduate, they should be placed with 12th graders, and graduate to 11th grade for the next year, and so on.
Kids are easier to teach when they’re older, since they’ve been in school so long already. The younger a kid is, the less they already know, both in subject matter and scholastic habits.
Teaching the youngest kids is tricky and a serious responsibility, which should be entrusted to those who have subtracted bits of that knowledge one by one, and faced the associated challenges.
Not all teachers want to teach at all grade levels. My sister specially is an early childhood educator. She wanted to be a kindergarten teacher, but ended up working for a private daycare. There is no way she would teach high school. One she does not know those subject to teach, two as said she is an early childhood teacher.
My high school English teacher was moved to the middle school and ended up retiring because she was a high school teacher, not a middle school teacher, it was not her wheel house teaching children of that age.