The thing is- if it was premeditated, then why did he fail to think about all of the things that gave him away?
- inadequate burial spot/plans/way to hide the bodies (shallow grave for one, tanks for the other two, tanks that would at some point be checked, especially since they were on his actual job, which LE would automatically check first)
- evidence left helterskelter- a sheet at the burial site, a bag at the burial site, her cell phone and other belongings and kids carseats, her car, EVERYTHING left inside their locked, videoed home.
- zero alibi
- no stage set/scene set up- if he had preplanned it, he would have had an actual plan, one that explained her absence, the girls' absences, one that left room open for them leaving voluntarily or being killed due to something else. Gas leak in the house, boat capsized and they drowned, fire, something. Not kill them, hide them, go to work and try to pretend it never happened.
- didn't call and re-arrange her medical appointment. Didn't use her phone to pretend to be her to throw her off (the whole discovery could have been avoided if CW had used SW's phone to text her friends/coworkers back, something simple like "I'm exhausted from all the travel, and need to rest this morning, will get back online in the morning, carry on without me! thanks dears!")
I personally don't think his daughter going to school was a factor at all. Why would it be? If you're hardcore evil enough to slaughter your whole family, why would it matter if it happened on a Sunday night before the first day of school or a Thursday night midway through the second semester?
I think whatever went down, whoever did what, it happened spontaneously, in the heat of the moment.