You should base it on what time they need to get up as well as how much sleep they need to be refreshed... If they don't have enough sleep, they'll be tired the next morning. But too much sleep can also make them feel groggy. Usually kids need 9-10 hours of sleep... So if you need to get them up by 6 to get them to school by 7, then getting them to bed by 8 is about perfect; 8pm-6am = 10 hours minus up to an hour just to get to sleep and they're still in that 9-10 hr range. If, on the other hand they aren't going to be getting up till 8 (with school starting at 9) then they could stay up till 9 or 10 and still be refreshed for the next day. Then again you may also want to wake them up more than just an hour before school starts, in which case they should get to bed earlier accordingly.
You may also want to gauge it with the sunset, esp. in the more northern areas where there is a greater shift in the amount of sun-lit hours. During the winter, you're probably safe to get them to bed by 7, but come the summer months, 7 may still seem bright as day, and the young kids won't understand why they have to go to bed "so early". Also if they have a strong circadian rhythm, they may be afflicted by SAD (Seasonal Affected Disorder) so that they'll be unable to get to sleep early, no matter how early they're sent to bed. If that is the case, you may need to invest in thick window covering so that their circadian rhythm won't be reset by the light and they can continue to get to sleep shortly after they are put to bed. You may also relent and allow them to stay up later in the summer months, so an 8 bedtime during the winter becomes a 9 bedtime come May...