Explanation
The consultant should recommend the school to build a Center of Excellence, because this can ensure that the expansion of their Salesforce environment is efficient, scalable, and will meet the needs of new and existing users. A Center of Excellence is a team or a group of teams that provide leadership, best practices, governance, and support for Salesforce within an organization. A Center of Excellence can help the school align their Salesforce strategy with their business goals, manage change and adoption, optimize performance and security, and foster innovation and collaboration. Creating a Trailmix for new users, granting System Admin access for each department leader, and encouraging users to post ideas on Chatter are not recommendations that can ensure that the expansion of the Salesforce environment is efficient, scalable, and will meet the needs of new and existing users. References:
https://www.salesforce.org/wp-content/uploads/2019/03/Education_Cloud_COE_Guide.pdf
https://trailhead.salesforce.com/en/content/learn/modules/center-of-excellence-basics