In Australia and in the International Baccalaureate (2 systems I have worked in), for better or worse, mathematics is assessed by criteria. This increases the importance of students to express their mathematical processing and problem solving strategies in their working out - as this is explicitly assessed as part of the criteria.
This is also a source of an infrequent, yet prevalent pedagogical-headache, which is the basis of my question - what strategies are there to effectively train senior high school students to compose legible, coherent and logical mathematical sequences leading to their solution?
This is a very common issue with students. There are several main concerns :
What you can do, is to change the motivations for them to write up their answers.
On Brilliant, I get a 75% submission rate when asking students who got a numerically correct answer to explain their full solutions. Note that this is not 'homework', nor are they required to submit answers in order to proceed on the site. When compared to math circles where this percentage is often close to 0, it is remarkable. [When I was training the singapore IMO team, we had to force the younger students to submit at least 1 solution as an entrance ticket to next week's session. That was the only way we were getting write-ups from them.]
The student solutions I receive on Brilliant are often well crafted, and heavily thought through. Several even start to learn Latex (though it is not required), in order to improve their presentation. Their solutions are then graded on a scale of incorrect, incomplete, correct, excellent. The best solution (up to my interpretation) is presented the following week for others to learn from. What I believe we are doing right, is to influence the motivations for students to write their answers.
I would suggest that you try adopting this method, and see if it works for your class. I would love to hear feedback about it.
Of course, we also offer them points to submit solutions. However, if bribery is so effective, I'd suggest that you simply bring cookies into class.