Teachers in the North Penn School District will fight the demotion of 36 of their own.   They say the cost cutting measure implemented by the school board is a violation of their contract and the school code.   The demotions followed the teacher's rejection of a six month pay freeze requested by the school board.   District officials say the union leadership withheld information about the proposed freeze from their membership before they voted it down and are calling for the resignation of union president Alan Malachowski.   Along with the demotions, the budget for the next school year approved by the school board Thursday also included the layoff of 13 support staff workers.