It sounds like you probably want to make similar documents over and over with the template facility. You make a "blank" document, get the settings the way you want them, then save as and choose "OmniOutliner 3 Template" as the file time (or just use Save As Template... if you have OmniOutliner Professional). Then whenever you need a grading sheet, or a class roster, or whatever you've made into templates, you open the template file and it makes you a new document based on the template.
As for setting the column width exactly, there is support in the Applescript dictionary for OmniOutliner to do that. The following script will prompt you for a value (in pixels) and set the width of all selected columns to that value. Is this better/easier than just dragging them all to be the right size? Depends on the severity of one's OCD, I think :-)
Code:
property defaultWidth : 100 -- default width, in pixels
tell application "OmniOutliner Professional"
tell front document
display dialog "Desired column width (in pixels)?" default answer defaultWidth buttons {"Cancel", "OK"} default button 2
set width of selected columns to (the text returned of the result) as integer
end tell
end tell