Observed something interesting here.  If you do the migration, but then run the old code against the new schema, and then try to insert a new row, you get a 500 error with this:

    Warning at /db/Winter2013/exams/save
    Field 'minutes_btw_attempts' doesn't have a default value

I doubt this is anything unique to this migration, just a comment that when we add a new column, when the model doesn't reflect this, you'll have potential errors like this on inserts for as long as the model and DB are out of sync.