I think we are in weird times with no routine of school so no enforced getting up "early".
There are many studies about sleep, that it isn't that beneficial to "catch up" on sleep by lying in on the weekend. It is the same as being on a different time zone every week. They also found that teenagers in countries without the technology we have manage to get out of bed much easier.
I think with the whole world in your hand in the form of a mobile phone, the ability to contact your friends without ringing a landline and getting their Dad, the TV shows that are available on demand and the fact that it no longer stops broadcasting programs at 11.30pm of whatever time it used to say goodnight all adds. I think we have all probably binged watched something, unwilling to stop the series, stayed up a bit later than we should and pay the price the next morning.
I have teenage sons, and I hate to say it but they usually get themselves up at 6.30am for school, they currently have their alarms set for 7am, they start their school work at 8.30 latest after showering, getting dressed and eating breakfast. They are in years 12 and 9. They are in bed for 10.
OP - the not doing school work is the issue and the possibility that other people in the house need to sleep as they have work the next day. Maybe come up with a compromise of a time you want them to be out of bed for.