You can provide to students a template for the answer.

By default, the system provides the student a blank field in the editor. You can also put any initial content. Furthermore, you can make the content to be a template for the answer, by using the custom attribute wrs:positionable="false/true".

See the MathML for the previous formula:

All the formula is marked wrs:positionable="false", so the student can not touch it, except a gap wanted to be filled in. This gap is made by <mrow wrs:positionable="true"/>, and it shows up as a green box to the student.