Blameless and not their fault are two different things.
The teachers and the unions, themselves are the very ones who complain the loudest when it comes to changing ANY of the terms set down by the department.
So in my opinion they are not blameless!
The standardised school year and school hours have been agreed by the employer , the employees trade unions , school representatives and parents organisations since 2004 and as such all have an input - if you personally have a problem with what was was agreed then I suggest that you target all the responsible parties.
Furthermore it appears that the standardised year and hours are unlikely to be changed in the forthcoming review at Easter as none of the negotiating parties are seeking an extension either of the standardised years or hours nor does it seem to be a case of concern for the majority of the public.
To the best of my knowledge the Unions have never challenged ANY aspect of the original 2004 agreement - if however you have facts to prove the contrary please don't hesitate to post same.
I would contend therefore that the teachers and their unions are indeed blameless as the standardised year and hours was mutually agreed by all interested parties and indeed adhered to by such parties.