write the whole essay first, then add in the TSs at the end
I think this is an excellent idea for beginning writers. Most kids know what they want to say--that's not usually the problem (if they're even halfway interested in the topic, that is). As I used to try and get across to my son, it isn't enough that the student knows the material--he has to be able to tell others what he knows. Same with peoples' thoughts--others have to be able to understand what they're trying to say.
I think it could be really, really helpful to some kids if they were given the freedom to put their thoughts down however they think them, and then be made to go back and pick out what goes together and what doesn't, and organize them. They could then create topic sentences, or perhaps might find that they had one already but hadn't realized it.

As to creating a mission statement: I should think you'd get far more "points" for having one that "matches" the report!

And then there are mystery novels: I've never written one, but I should imagine that most authors figure out first how the mystery is going to be resolved, rather than build up and up to it and THEN try to think of a way out.