I have 3 rules.
1) General school day rule (8:30pm)
All school going kids should be in bed by 8:30pm. Works really well, they get up by 7am by themselves the next they.
2) General school day "rewards" minutes (9pm)
An extra 30 minutes of play time are granted if a very hard/long homework was completed and/or an A was obtained on an exam. They still wake up by 7am next day and really promotes study.
3) Weekend rule (9pm)
An extra 30 minutes are granted if there's no school the next day.
4) Stacking Rules (9:30pm)
On some Fridays, a test result comes in, granting "reward" minutes on a day where "weekend" minutes were already granted. On such cases, depending on activities planned for the next day, I might allow stacking of the minutes, moving bed time until 9:30pm.
I am very strict on this. How about you?