Not sure why "select all" isn't working. However what I used to do in your situation is to create a new course which is a clone of the old one without the students and with a new name (e.g. math100_fall14). This is easy to do:
In the Hmwwk Sets Editor, select Export, Select all sets, Take Action. This saves set definition files for all your sets. Then in the Admin Course, create a new course but in the Copy templates from: box, select your old course. Then in the new course, in the Hmwwk Sets Editor, select Import, etc.
That way you always have your old course in case you want it for some reason but maybe you want to turn off access to it from the old students.