Hi Matf. Welcome (I'm new here too). :)
The phrase fall in line does not sound natural to me (at least, I am not aware of any context in which it is common to express the idea in quite that way).
In an army context, the command Fall in! is an instruction for soldiers to get into formation. The "following" would then occur when they started marching, so your phrase still wouldn't quite work as it is written.
- For Teachers