We can't help you make a tool, per se, as in a custom program, but the way to do this in a professional office environment would be through Active Directory Group Policy.
As this article shows, you can set all of the power policy settings such as when to hibernate after being idle, etc., on all your computers in your organization in a consistent and maintainable fashion through Group Policy.
It may not have the granularity that you were hoping for (i.e. 'computer must hibernate at 10:30pm', etc.) but anything beyond what you can do with Group Policy would involve a custom solution, probably something that you wrote, or at least some scripts tacked on with Task Scheduler (you can also deploy scripts through Group Policy,) etc... and since you said you have 'a lot of computers,' that would be a maintenance nightmare in my opinion versus Group Policy.
The power management policies are located in Power Management, under
Computer Configuration | Administrative Templates | System | Power
Also you might want to remember to "Allow wake timers" in the computer's power plan if you intend to wake it up automatically at a certain time. A scheduled task has the ability to wake a computer from a sleep state, if you configure it to and you've allowed that in your power policy.
And finally, as Jscott points out, most hardware these days has Wake-On-Lan "magic packet" type functionality if that can be of use to you.
So these are pretty much the tools you have at your disposal without getting into custom development land.