diff --git a/ucs-school-lib/python/models/group.py b/ucs-school-lib/python/models/group.py
index f0240bc..ad29b8c 100644
--- a/ucs-school-lib/python/models/group.py
+++ b/ucs-school-lib/python/models/group.py
@@ -240,6 +240,12 @@ class SchoolClass(Group, _MayHaveSchoolPrefix):
 		ret['name'] = self.get_relative_name()
 		return ret
 
+	@classmethod
+	def get_class_for_udm_obj(cls, udm_obj, school):
+		if not cls.is_school_class(school, udm_obj.dn):
+			return  # is a workgroup
+		return cls
+
 
 class WorkGroup(SchoolClass, _MayHaveSchoolPrefix):
 
diff --git a/ucs-school-lib/python/models/user.py b/ucs-school-lib/python/models/user.py
index 104b75d..24c64a8 100644
--- a/ucs-school-lib/python/models/user.py
+++ b/ucs-school-lib/python/models/user.py
@@ -253,45 +253,35 @@ class User(UCSSchoolHelperAbstractClass):
 			self.password = old_password
 		return success
 
-	def do_modify(self, udm_obj, lo):
-		self.create_mail_domain(lo)
-		self.password = self.password or None
-		mandatory_groups = self.groups_used(lo)
-		for group_dn in udm_obj['groups'][:]:
-			# User might have been removed from school(s), remove class
-			# and workgroup memberships for those. Leave other group types
-			# (e.g. global groups) untouched.
-			logger.debug('Checking group %s for removal', group_dn)
-			if group_dn not in mandatory_groups:
-				logger.debug('Group not mandatory! Part of a school?')
-				try:
-					school_group = Group.from_dn(group_dn, None, lo)
-				except noObject:
-					logger.debug('No. Leaving it alone...')
-					continue
-				# Bug 41907:
-				# SchoolClass.from_dn does not throw WrongModel if no class or workgroup.
-				# But self_is_class() and self_is_workgroup() return correct values.
-				if school_group.self_is_class():
-					classes = self.school_classes.get(school_group.school, [])
-					remove = school_group.name not in classes and school_group.get_relative_name() not in classes
-				elif school_group.self_is_workgroup():
-					remove = school_group.school not in self.schools
-				else:
-					logger.debug('Group is no school class or workgroup. Leave it untouched.')
-					continue
-				logger.debug('Yes, part of %s!', school_group.school)
-				if remove:
-					logger.debug('Removing it!')
-					udm_obj['groups'].remove(group_dn)
-				else:
-					logger.debug('Leaving it alone: Part of own school and either non-school class or new school classes were not defined at all.')
-		for group_dn in mandatory_groups:
-			logger.debug('Checking group %s for adding', group_dn)
-			if group_dn not in udm_obj['groups']:
-				logger.debug('Group is not yet part of the user. Adding...')
-				udm_obj['groups'].append(group_dn)
-		return super(User, self).do_modify(udm_obj, lo)
+		def do_modify(self, udm_obj, lo):
+			self.create_mail_domain(lo)
+			self.password = self.password or None
+			mandatory_groups = self.groups_used(lo)
+			for group_dn in udm_obj['groups'][:]:
+					logger.debug('Checking group %s for removal', group_dn)
+					if group_dn not in mandatory_groups:
+							logger.debug('Group not mandatory! Part of a school?')
+							try:
+									school_class = SchoolClass.from_dn(group_dn, None, lo)
+							except noObject:
+									logger.debug('No. Leaving it alone...')
+									continue
+							logger.debug('Yes, part of %s!', school_class.school)
+							if school_class.school not in self.school_classes:
+									continue  # if the key isn't set we don't change anything to the groups. to remove the groups it has to be an empty list
+							classes = self.school_classes[school_class.school]
+							remove = school_class.name not in classes and school_class.get_relative_name() not in classes
+							if remove:
+									logger.debug('Removing it!')
+									udm_obj['groups'].remove(group_dn)
+							else:
+									logger.debug('Leaving it alone: Part of own school and either non-school class or new school classes were not defined at all')
+			for group_dn in mandatory_groups:
+					logger.debug('Checking group %s for adding', group_dn)
+					if group_dn not in udm_obj['groups']:
+							logger.debug('Group is not yet part of the user. Adding...')
+							udm_obj['groups'].append(group_dn)
+			return super(User, self).do_modify(udm_obj, lo)
 
 	def do_school_change(self, udm_obj, lo, old_school):
 		super(User, self).do_school_change(udm_obj, lo, old_school)