diff --git a/server/vbv_lernwelt/assignment/services.py b/server/vbv_lernwelt/assignment/services.py
index 11e29a9d..c2708786 100644
--- a/server/vbv_lernwelt/assignment/services.py
+++ b/server/vbv_lernwelt/assignment/services.py
@@ -173,7 +173,7 @@ def update_assignment_completion(
                 acl.completion_data[key].update(task_data)
         acl.save()
 
-        if completion_status == "SUBMITTED":
+        if completion_status == AssignmentCompletionStatus.SUBMITTED:
             learning_content = assignment.learningcontentassignment_set.first()
             if learning_content:
                 mark_course_completion(
diff --git a/server/vbv_lernwelt/course_session/admin.py b/server/vbv_lernwelt/course_session/admin.py
index b0970a8e..da09cef2 100644
--- a/server/vbv_lernwelt/course_session/admin.py
+++ b/server/vbv_lernwelt/course_session/admin.py
@@ -9,7 +9,12 @@ from vbv_lernwelt.course_session.models import (
 @admin.register(CourseSessionAttendanceCourse)
 class CourseSessionAttendanceCourseAdmin(admin.ModelAdmin):
     # Inline fields are not possible for the DueDate model, because it is not a ForeignKey relatoion.
-    readonly_fields = ["course_session", "learning_content", "due_date"]
+    readonly_fields = [
+        "course_session",
+        "learning_content",
+        "due_date",
+        "attendance_user_list",
+    ]
     list_display = [
         "course_session",
         "learning_content",
diff --git a/server/vbv_lernwelt/course_session/models.py b/server/vbv_lernwelt/course_session/models.py
index 7cd6738c..3408ae32 100644
--- a/server/vbv_lernwelt/course_session/models.py
+++ b/server/vbv_lernwelt/course_session/models.py
@@ -1,5 +1,6 @@
 from django.db import models
 from django.utils.translation import gettext_lazy as _
+from django_jsonform.models.fields import JSONField as JSONSchemaField
 
 from vbv_lernwelt.assignment.models import AssignmentType
 from vbv_lernwelt.duedate.models import DueDate
@@ -32,7 +33,24 @@ class CourseSessionAttendanceCourse(models.Model):
     # because the attendance list is more of a snapshot of the current state
     # we will store the attendance list as a JSONField
     # the important field of the list type is "user_id"
-    attendance_user_list = models.JSONField(default=list)
+    ATTENDANCE_USER_LIST_SCHEMA = {
+        "type": "array",
+        "items": {
+            "type": "object",
+            "properties": {
+                "user_id": {
+                    "type": "number",
+                    "required": True,
+                },
+                "email": {"type": "string"},
+                "first_name": {"type": "string"},
+                "last_name": {"type": "string"},
+            },
+        },
+    }
+    attendance_user_list = JSONSchemaField(
+        default=list, schema=ATTENDANCE_USER_LIST_SCHEMA
+    )
 
     def save(self, *args, **kwargs):
         if not self.pk:
diff --git a/server/vbv_lernwelt/course_session/services/attendance.py b/server/vbv_lernwelt/course_session/services/attendance.py
index 79ffb62b..22802943 100644
--- a/server/vbv_lernwelt/course_session/services/attendance.py
+++ b/server/vbv_lernwelt/course_session/services/attendance.py
@@ -20,7 +20,9 @@ def update_attendance_list(
     attendance_course: CourseSessionAttendanceCourse,
     attendance_user_list: List[AttendanceUser],
 ):
-    user_id_list_before = [u["user_id"] for u in attendance_course.attendance_user_list]
+    user_id_set_before = set(
+        [u["user_id"] for u in attendance_course.attendance_user_list]
+    )
 
     result_user_list = []
     for attendance_user in attendance_user_list:
@@ -52,14 +54,13 @@ def update_attendance_list(
     attendance_course.attendance_user_list = result_user_list
     attendance_course.save()
 
-    user_id_list_after = [u["user_id"] for u in attendance_course.attendance_user_list]
+    user_id_set_after = set(
+        [u["user_id"] for u in attendance_course.attendance_user_list]
+    )
 
-    user_id_list_removed = [
-        user_id for user_id in user_id_list_before if user_id not in user_id_list_after
-    ]
+    user_id_set_removed = user_id_set_before - user_id_set_after
 
-    for user_id in user_id_list_removed:
-        # FIXME: create completion for every user in attendance_user_list
+    for user_id in user_id_set_removed:
         u = User.objects.filter(id=user_id).first()
         if u is not None:
             mark_course_completion(