A tenured faculty member shall not be dismissed from his appointment except for sufficient cause, nor shall a faculty member who holds a probationary faculty appointment be dismissed prior to the dates established in the written terms of his appointment except for sufficient cause which is of a substantial nature and is not frivolous or inconsequential as determined by the board of trustees, and when it is determined by the board of trustees that the termination of his employment would be in the best interests of the community college district.