Neue Funktion Schüler austragen statt aus Kurs löschen.

LehrerApp.Desktop.Tests/Fakes.cs

@@ -21,8 +21,12 @@ public class FakeMemberships(List<GroupMembership> all) : IGroupMembershipReposi
     public List<GroupMembership> GetByGroup(Guid groupId) => all.Where(m => m.GroupId == groupId).ToList();
     public GroupMembership? GetByStudentAndGroup(Guid studentId, Guid groupId) =>
         all.FirstOrDefault(m => m.StudentId == studentId && m.GroupId == groupId);
-    public void Save(GroupMembership membership) { }
-    public void Delete(Guid id) { }
+    public void Save(GroupMembership membership)
+    {
+        all.RemoveAll(m => m.Id == membership.Id);
+        all.Add(membership);
+    }
+    public void Delete(Guid id) => all.RemoveAll(m => m.Id == id);
 }
 
 public class FakeSessions(List<ParticipationSession> all) : IParticipationSessionRepository

LehrerApp.Desktop.Tests/ReportGradeCalculationTests.cs

@@ -22,7 +22,8 @@ public class ReportGradeCalculationTests
         var grading = new GradingService();
 
         return new ReportGradeDialogViewModel(grades, exams, results, students, memberships,
-            schemes, reportGrades, grading, GroupId, GroupType.Class, GradingSystem.Grades1To6, "Testgruppe");
+            schemes, reportGrades, grading, GroupId, GroupType.Class, GradingSystem.Grades1To6,
+            "Testgruppe", "2025/26");
     }
 
     private static (Exam Exam1, Exam Exam2, FakeExams Exams, FakeResults Results) BuildExams()

LehrerApp.Desktop.Tests/WithdrawStudentDialogViewModelTests.cs

@@ -0,0 +1,39 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Desktop.ViewModels.Groups;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public class WithdrawStudentDialogViewModelTests
+{
+    [Fact]
+    public void InitialisiertMitHeutigemDatumUndSpeichertAustritt()
+    {
+        var membership = new GroupMembership { JoinedAt = new DateOnly(2025, 9, 1) };
+        var memberships = new FakeMemberships([membership]);
+        var vm = new WithdrawStudentDialogViewModel(
+            memberships, membership, "Anna Beispiel", "7a", new DateOnly(2025, 10, 5));
+
+        Assert.Equal(new DateOnly(2025, 10, 5), DateOnly.FromDateTime(vm.SelectedDate!.Value.LocalDateTime));
+
+        vm.SaveCommand.Execute(null);
+
+        Assert.True(vm.WasSaved);
+        Assert.Equal(new DateOnly(2025, 10, 5), membership.LeftAt);
+    }
+
+    [Fact]
+    public void VerhindertAustrittVorEintritt()
+    {
+        var membership = new GroupMembership { JoinedAt = new DateOnly(2025, 9, 1) };
+        var vm = new WithdrawStudentDialogViewModel(
+            new FakeMemberships([membership]), membership, "Anna Beispiel", "7a");
+        vm.SelectedDate = new DateTimeOffset(2025, 8, 31, 0, 0, 0, TimeSpan.Zero);
+
+        vm.SaveCommand.Execute(null);
+
+        Assert.False(vm.WasSaved);
+        Assert.Null(membership.LeftAt);
+        Assert.NotEmpty(vm.DateError);
+    }
+}