Include the Haml gem.
Change erb to haml.
Fix a indentation bug.
Fix a but. Students can't destroy their lessons.
Fix a bug, If used jQuery document ready with turborlinks then detection page:load at the same time.
Remove a useless line.
Support the mp3 and the ogg format.
Change the datetime display format.
Change the datetime picker plugin to jquery.datetimepicker.
Use the chosen jquery plugin for choosing students.