So I did use writing #alttext as the task in an exam. Describing pictures is something we do anyway, even in higher classes.
Of course students had never heard of alt-text, so I printed part of @alttexthalloffame 's text on https://alttexthalloffame.org.
The students' texts turned out more focused and relevant than usually. I believe that is because the situation makes sense and lets them write _for_ somebody.
part of the exam (pdf): https://wob.itslearning.com/data/200590/409605/exam%20listening%20sport%2c%20writing%20alttext.pdf