Formalizing and its challenges

I've really been feeling the tension recently between emphasizing creativity, different ways of thinking, innate mathematical processes that are genuinely student-driven and the type of formal math notation and expression that are needed in order for us to have a common language and to be able to demonstrate our understanding to people outside of our community.

This is the first year in a long time that I'm working with students (7th graders) whose almost entire math learning experience has been rich and validating of the importance of expressing their thoughts and ideas in ways that made sense to them. They have done a lot of open projects and pattern investigations. As a result, they are exceedingly curious and creative in their approaches. They are not into answer-getting, they listen to the ideas of others, and they demonstrate really cool insights and ways of thinking. Having said that, their notation and formalizing of thoughts is ghastly. Their work is just numbers and symbols all over the place, a very personal record that somewhat makes sense to the student writing it, but is incomprehensible to anyone else. Equal signs are placed willy nilly, variables are used with little rhyme or reason to sometimes mean one stage and sometimes mean the previous/next stage, the progression of thought skips blithely around the page in seemingly random directions.

So I'm in a position where I know that I need to teach some formalization of process, some common notation and standardization of the way that we communicate our thinking and show our work. But I want to do this in a way that doesn't destroy the freedom of thought that has been carefully cultivated by their previous teachers, their ownership of mathematics as personal expression. Every time I ask a student to show their work in the very specific, standard way, just like all the other round pegs, I feel a little bit like I'm crushing something wild and pure and free.

It's a math fairy in its natural habitat! Let it run wild and free!

Help me out, teachers of younger students. How do you help students channel their approaches without crushing their spirit? How do I know how much to push formal notation? Our high school does not have an Algebra 1 class so by the end of 8th grade, they are supposed to have learned the equivalent of a standard Algebra 1 class. In my previous school, formal and precise approaches were held in very high regard and students bought in and didn't question it. I received my yearly package of students, some of who maybe weren't so amazing at formalizing their thinking, but were definitely aware that this was a goal for which to strive and gave a reasonably good effort to make it happen. Not so here. I feel like I need to be fully confident and able to justify to these students (and their parents) that what I'm doing is for their best development as students of mathematics. And clearly, I have some doubts at the moment. 

If you teach middle school math, I'd love your thoughts and feedback. How do you get buy in to formalization? My approach so far has been to first let them tackle problems intuitively and then try to demonstrate how to convert that into a more formal way, but their response so far has been a bit of

I feel like I can create some need and urgency to communicate more clearly by having them read and edit each others' work, but that won't likely get them to writing in the standard ways that the rest of the math world shows their thinking. And how to approach formal ways of writing without narrowing their thinking and reducing ownership? Or is that a conflict that's inevitable and just part and parcel of continuing in one's studies as a student of mathematics?