Final answer:
Encouraging students to wear their former school colors, increasing short-term contact, and labeling one group as the 'home' group are effective ways to promote unity between former rival high schools.
Step-by-step explanation:
One of the most effective ways to promote unity between former rival high schools is to encourage students to wear their former school colors for the first year. This can help to create a sense of pride and identity among the students from each school. Additionally, finding ways to increase short-term contact between the former rivals can help to break down barriers and foster relationships. This could involve organizing joint activities, clubs, or events that require collaboration between students from both schools. Another effective approach is to label one group as the 'home' group and have them take primary responsibility for the leadership positions. This can help to create a sense of ownership and equal representation among both schools.