What is alma mater,It indicates your school or college.Definitely speaking,it represent your former school,i.e. where you have graduated from,yes it's means your mother school,but it have been used more often.In fact,alma mater is Latin language,however it has enjoyed great popularity among us graduates.So it is better to displace the ''mother school'' in your eassy,for it's more  common.

Alumni means male graduates,likewise,alumna is the female graduates.