correct word/s to fill in the blank

[kohyoongliat]

The primary school period is the time when/ in which/during which parents should begin to teach their children to make some decisions on their own.

I think 'during which' cannot be used here, but the other two options are fine.

Am I correct?

Thanks in advance.

[Mister Micawber]

.
During which is fine-- in fact, that would be my choice; in which and when and that are also OK.
.