your first step should start from the Schools and probably from the retirees who had worked and had experience with many administrations of the School as well as to get information from old students, thus information can also be source from the non academy of the school but present and formal staffs.

Start with National Native American Boarding School Healing Coalition. They have a lot of information on their website. There will be organizations and websites for both United States and Canada North America), and I haven't looked yet, but I wouldn't be surprised if there are sites with information about Central America. It's unlikely that you will be able to speak directly with someone who attended a boarding school because it was extremely painful time for them that has led to generational trauma. The Coalition above does have some stories they have collected. There are several Native organizations who are collecting the stories from those who attended the schools. You can also research the colonizer perspective through historical newspaper articles.

You can also find information within the publicly accessible files at the National Archives within the Bureau of Indian Affairs, you will need to search the school if there is available information for the public to access. I found a sample (link below) for you that shows a photo of some students of the school. You will want to search for Indian School, not necessarily a boarding school going forward. This link shows what category these are in as well. Please reach out if you need further assistance.

https://catalog.archives.gov/id/313190192

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