Whether it's a septic system "or some other similar scenario", I don't believe she was dumped, or buried in a fashion that would allow a skull to appear behind a school. If you don't want someone found, a grave where it doesn't belong would be pretty risky. I also can't imagine that they would choose a septic system type scenario on occupied property with routine activities of daily life.
I've always thought they had a good plan, some help, and a little bit of luck, and this explains why she hasn't been found. I don't think the plan included a time intensive grave-digging in the wee hours, or even prior, unless there was some access to a secluded area the M's controlled and could be certain would not be noticed or happened upon during their activity. Behind a school or in a human-traveled and animal inhabited forest don't seem like good options to me.
This was very personal to TM. I believe she made the disposal of her victim's body as personal as she did her killing, and I don't think a random site, or a site that posed potential discovery by parties out of the control of TM would be her choice. JMO. I could be entirely wrong.
Personally, I've always found it remarkable that the M's arrived at and left the landing so swiftly, went back in the direction of their own home, then their activity and the evidence trail seems to have abruptly ended.
Again, if Heather was killed at the landing, then what happened? My question isn't a challenge to you personally, it's a larger question as to how all this fits together, and in addition, allows for such successful disposal of the victim when the picture that's been painted, or suggested, is one of the M's heading back toward their home and not showing up elsewhere on camera or in other circumstances that illuminate the trail to and from what happened that morning.