We are writing a textbook that has content for the student and recommendations directed to the teacher, something like this:
How can we sum $\frac{3}{7}$ with $\frac{7}{3}$?
\begin{teacher}
A common mistake here is for the student to answer $\frac{10}{10}$.
\end{teacher}
This is how we do it...
So far so good. The problem lies on the way in which I want this text to be rendered.
We wanted to have the student text inside a minipage, without any text to the teacher inside. Then we want the teacher's material to surround the minipage, like in the following figure:
Visually, it looks similar to the question Making the text fit in a specific space in Latex, but the main problem we face is forcing the two texts to flow together, i.e. the suggestion for the teacher to appear close to the relevant exercise for the student.
There are several packages that could help, such as parcolumns. But they all seem to fail to sync the texts.
It would be ok to have to compile the text twice or to write metadata to the .aux file, but how would I go about this?

tufte-bookclass could be useful for you, as it is designed to place a lot of contents in a wide margin. Runningtexdoc sample-bookwill show you a sample. – Fran Nov 09 '17 at 18:10