Comment 4 for bug 1534130

No, reinstall is not what I suggest, that's what a user did and it somehow helped.

You can try reinstalling only the problematic package

$ sudo apt-get install --reinstall python-zope.html

and start/stop schooltool.

What versions of the packages do you have: python-schooltool, python-schooltool.gradebook, python-zope.html? You can do this with a command, that also lets determine where that package came from.

$ apt-cache policy python-zope.html
python-zope.html:
  Installed: 2.2.0-0ubuntu11
  Candidate: 2.2.0-0ubuntu11
  Version table:
 *** 2.2.0-0ubuntu11 0
        500 http://lt.archive.ubuntu.com/ubuntu/ wily/universe amd64 Packages
        100 /var/lib/dpkg/status

I suspect from Bug #1533147 you reported earlier, the problem is that you have a local copy of an old version somewhere, and it gets picked up first.

Does this output anything?

$ grep ckeditor /etc/schooltool/standard/plugins