Comment 12 for bug 1272181

It is still trying to install a month-old snapshot, python-schooltool (1:2.6.2~dev.201401061333~ubuntu12.04.1)

Means the repositories are not properly configured.

For some reason, the command that should have removed the trunk PPA (add-apt-repository --remove) did not work.
So remove it from sources by hand.

  $ sudo rm /etc/apt/sources.list.d/schooltool-owners-trunk-precise.list

You need to add SchoolTool dev PPA that contains 2.6

  $ sudo add-apt-repository ppa:schooltool-owners/dev
  $ sudo apt-get update

To see what versions of a package are available, use apt-cache policy

$ apt-cache policy python-schooltool
python-schooltool:
  Installed: 1:2.6.2~dev.201401231446~ubuntu13.10.1
  Candidate: 1:2.6.2~dev.201401231446~ubuntu13.10.1
  Version table:
 *** 1:2.6.2~dev.201401231446~ubuntu13.10.1 0
        100 /var/lib/dpkg/status
     1:2.6.1.2-0ubuntu2~saucy1 0
        500 http://ppa.launchpad.net/schooltool-owners/ppa/ubuntu/ saucy/main amd64 Packages
     1:2.6.0-0ubuntu1 0
        500 http://lt.archive.ubuntu.com/ubuntu/ saucy/universe amd64 Packages

For you it will be different, but the line above "100 /var/lib/dpkg/status" shows currently installed version, and the following lines show version and source. I also have a 2.6.2~dev version, but it's not available from any sources any more.

Then install the latest version that is now available (2.6.1.2)

  $ sudo apt-get install schooltool/precise python-schooltool/precise

There will be a warning "The following packages will be DOWNGRADED", answer Yes.

If other plugins have a 2.6.2~dev version, you need to downgrade them as well:

  $ sudo apt-get install python-schooltool.gradebook/precise \
      python-schooltool.lyceum.journal/precise \
      python-schooltool.intervention/precise python-schooltool.cando/precise

Maybe your package databases are broken after unsuccessful upgrades. There are two commands to fix unfinished installs

  $ sudo dpkg --configure -a
  $ sudo apt-get -f install

They do nothing if all is good.