School-Teacher breach of contract?

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I have taught at a private middle school in MA for 6 years. I signed a contract in April 08 at 88% of full time pay. This was for a specific set of classes and responsibilities. They signed, I signed. It was the same % and duties as the previous year. My base job 6 years ago started at 60% the first year and has increased over the years as I took on more classes. So my current 88% is based on the previous 6 years and all of those contracts too.

In July 08, I received notice from the headmaster that there was a mistake in my percentage or duties- the school had either given me too high a percentage in 2007-08 or not enough classes/responsibilities.

So now they want me to:
1) accept more duties/classes this year for same 88%
or
2) take a pay cut/percentage cut to the amount they think is appropriate to my work load

Can they do this when we have already signed the contract agreeing on terms for this school year? It sounds like a breach of contract to me.