Update guide_for_teachers.md

[katrite]

Proposed changes

Briefly describe the changes you've made here. Remember to add a link to the preview page of your branch.

Checklist before requesting a review

• I have followed the instructions in the Contributing and Styleguide documents.
• My pull request passes the automated tests. If not, visit pull requests at Travis CI and have a look at the job log.
• I have included a link to the appropriate preview page (select your branch from the list).

[rkronberg]

Looks like this has been been taken care of by #2546