While Dec will work, I also think that an Aug UY might be a better option.
With August, you have flexibility to cover all the times you mention, but most important, still have time to rebook trips if you did have to cancel.
For example, if you had an April trip planned wth borrowed points and had to cancel, you would still be able to rebook that trip during July (assuming this is one of the months you have off as a teacher).
However, if you have a Dec UY and had an Aug trip with borrowed points that had to be cancelled, would you be able to rebook a trip during September through November. Since you teach, it seems like it would be difficult. Plus, that time of year is really busy and getting a last minute reservation might be more difficult.
Of course, an Aug UY won't get you 2009 UY points if you are going direct through Disney.