It's purely a game of luck, that's probably why there's no demotion (even though it says there is in the rules).
I did the highest tasks and came #60, I would be pissed if I got demoted after that.
If you always do the highest value tasks, eventually you'll make the top10 and get a reward/promoted.
In order for that to happen though, you probably have to spend a lot of SimCash to finish the airport deliveries fast, since that's where the 5,000 point tasks are usually assigned. I've already done 2 this League, but my other tasks sucked, my score isn't competitive so far.