Springfield’s bishop said the Cathedral and Holyoke Roman Catholic high schools will be merged for their long-term financial stability, but the location of a new school has not been decided. Bishop Mitchell Rozanski made the announcement Monday as the Western Massachusetts diocese considers whether to rebuild at the Springfield site where Cathedral High was severely damaged in a 2011 tornado. Many supporters, including Springfield’s mayor, are urging that the school be rebuilt there. Rozanski said the location is among those being considered. He hopes a new school can be built within 2½ years using insurance money and federal disaster aid.