SwiftData deleteRule .cascade

I made simple school & student model with RelationShip

import SwiftUI
import SwiftData

@Model final class School {
    var name: String
    
    @Relationship(deleteRule: .cascade, inverse: \Student.school)
    var studentList: [Student] = [Student]()
    
    init(name: String) {
        self.name = name
    }
    
}

@Model final class Student {
    var name: String
    var school: School?
    
    init(name: String) {
        self.name = name
    }
}

~

struct SchoolView: View {
    @Environment(\.modelContext) private var modelContext
    @Query(sort: \School.name) var schools: [School]
    @Query(sort: \Student.name) var students: [Student]
    
    var body: some View {
        NavigationStack {
            List {
                ForEach(schools) { school in
                    NavigationLink {
                        StudentView(school: school)
                    } label: {
                        HStack {
                            Text(school.name)
                            Spacer()
                            Text("\(school.studentList.count)")
                        }
                    }
                    .swipeActions {
                        Button(role: .destructive) {
                            deleteSchool(school)
                        } label: {
                            Label("Delete", systemImage: "trash")
                        }
                    }
                }
                .onDelete(perform: deleteSchools(at:))
            }
            .navigationTitle("Home")
            .listStyle(.plain)
            .toolbar {
                ToolbarItem(placement: .topBarTrailing) {
                    Button {
                        addSchool()
                    } label: {
                        Image(systemName: "plus")
                    }
                }
                
                ToolbarItem(placement: .topBarLeading) {
                    VStack(alignment: .leading) {
                        Text("School: \(schools.count)")
                        Text("Student: \(students.count)")
                    }
                }
            }
        }
    }
    
    private func randomString() -> String {
        let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
        return String((0..<5).map { _ in letters.randomElement()! })
    }
    
    private func addSchool() {
        let newSchool = School(name: randomString())
        modelContext.insert(newSchool)
    }
    
    private func deleteSchools(at offsets: IndexSet) {
        offsets.map { schools[$0] }.forEach(deleteSchool)
    }
    
    private func deleteSchool(_ school: School) {
        modelContext.delete(school)
        try? modelContext.save()
    }
    
    
}

struct StudentView: View {
    @Environment(\.modelContext) private var modelContext
    
    var school: School
    
    var body: some View {
        List {
            ForEach(school.studentList) { student in
                Text(student.name)
            }
            .onDelete(perform: deleteStudents(at:))
        }
        .navigationTitle("Home")
        .navigationBarTitleDisplayMode(.inline)
        .listStyle(.plain) 
        .toolbar {
            ToolbarItem(placement: .topBarTrailing) {
                Button {
                    addStudent()
                } label: {
                    Image(systemName: "plus")
                }
            }
        }
    }
    
    private func addStudent() {
        let newSchool = Student(name: randomString())
        modelContext.insert(newSchool)
        newSchool.school = school
        school.studentList.append(newSchool)
    }
    
    private func randomString() -> String {
        let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
        return String((0..<5).map { _ in letters.randomElement()! })
    }
    
    private func deleteStudents(at offsets: IndexSet) {
        withAnimation {
            offsets.forEach {
                let student = school.studentList[$0]
                modelContext.delete(student)
                try? modelContext.save()
            }
        }
    }
}

I try my best to follow the SampleTrips app from Apple developer.

When I delete one of the schools, then all the students in the school should be deleted. But the student count never changes. Deleting the student itself works well - the student count decreases.

Thank you!