调整

app/views/courses/search_users.json.jbuilder

@@ -2,8 +2,8 @@ json.users do
   json.array! @users do |user|
     json.id user.id
     json.name user.real_name
-    json.student_id user.user_extension.try(:student_id)
-    json.school_name user.user_extension.school.name
+    json.student_id user&.student_id
+    json.school_name user&.school_name
     json.added @course.course_member?(user.id, 4)
   end
 end