It depends on what you have to do for your job.
Some people have to 'be there' during certain hours, for team meetings, answering client / customer / public 's call, delivering training, delivering presentations, supervising colleagues, etc etc.
Others, can fit their (in your case) 6 hours in more flexibly to suit how they work best.
So for me (supposed to work 7.5 hours a day) I will often stop at about 10.30 and hang out some washing or prep some dinner for the slow cooker, or to marinade or whatever, which I can, because I stop the clock at the same time, and that isn't part of my working hours.
So, if other people aren't reliant on you being there at the computer / end of the phone, then you can walk the dog / put the dinner on / go for a run as you can spread your 6 hours across a 9 or 10 hour period.
But it sounds as if you don't want to work after 3, so therefore your choice is to work 6 hours straight, or start earlier, and have your dh do the drop off at school if you are doing the pick up.