Which time should a 12 year old go to bed during the school week?

Varies: A 12 year old will need a minimum of 9 hours of sleep/day during his/her growing phase and more if demanding physical or study activities are present.A kid that needs to get up at 5 for chores or specific workouts would need a different bedtime than one arising at 7. It is the duration of sleep not the bedtime that determines a reasonable schedule.