else {
        RecordBook *next, *prev = listOfStudents;
        current = listOfStudents;

        while (current->next && current->index != pos) {
            prev = current;
            current = current->next;
        }

        next = current->next;
        prev->next = next;

        //!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
        ***//Вот здесь срабатывает ошибка***
        delete current;
        current = NULL;
    }

#include <iostream>
#include <stdio.h>
#include <string>

using namespace std;


class RecordBook {
public:
    void setName(string);
    string getName();
    void setLastName(string);
    string getLastName();
    void setScores(int, int, int, bool, bool, bool);
    void showScores();

private:
    string name;
    string lastName;

    int Math;
    int MathAnalysis;
    int Programming;

    bool English;
    bool PhysEducation;
    bool History;

    //Navigation
    size_t index = 0;
    RecordBook *next = nullptr;

    friend class Group;


};

class Group {
public:
    //Добавить студента
    void addStudent(string, string, int=5, int=5, int=5, bool=true, bool=true, bool=true);
    //Найти студента и вывести оценки
    void findStudent(string);
    //Вставить зачетную книжку в заданный индекс
    void insert(RecordBook &, size_t = 0);
    //Удалить зачетную книжку по заданному индексу
    void erase(size_t = 0);

private:
    //Динамический список студентов, который указывает на последнего студента
    RecordBook *listOfStudents;
    //Количество студентов
    size_t countOfElements = 0;

    //Создать новый узел для студента в динамическом списке
    void createOfNode(RecordBook &);
    //Обновить индексы динамического списка
    void updateOfIndex();

} mx101;

int main() {
    setlocale(0, "");

    mx101.addStudent("Al", "As");
    mx101.addStudent("Petya", "Petrov");

    RecordBook student;
    student.setName("vasya");
    student.setLastName("XXX");
    student.setScores(2, 3, 4, 1, 1, 0);

    mx101.insert(student, 1);
    mx101.erase(1);

    return 0;
}

void RecordBook::setName(string name1) {
    name = name1;
}

string RecordBook::getName() {
    return name;
}

void RecordBook::setLastName(string lastName1) {
    lastName = lastName1;
}

string RecordBook::getLastName() {
    return lastName;
}

void RecordBook::setScores(int M, int MA, int pr, bool En, bool Ph, bool Hi) {
    Math = M;
    MathAnalysis = MA;
    Programming = pr;

    English = En;
    PhysEducation = Ph;
    History = Hi;
}

void RecordBook::showScores() {
    printf("Name: %s, Last Name: %sn", name.c_str(), lastName.c_str());
    printf("Math: %d, Math analysis: %d, Programming: %dn", Math, MathAnalysis, Programming);
    printf(English ? "English: passedn" : "English: didn't passedn");
    printf(PhysEducation ? "PhysEducation: passedn" : "PhysEducation didn't passedn");
    printf(History ? "History: passedn" : "History: didn't passedn");
    cout << endl;
}

void Group::addStudent(string name, string lastName, int a, int b, int c, bool d, bool e, bool f) {
    RecordBook student;
    student.setName(name);
    student.setLastName(lastName);
    student.setScores(a, b, c, d, e, f);

    createOfNode(student);
}

void Group::createOfNode(RecordBook &elem) {
    RecordBook *current = new RecordBook;

    if (countOfElements == 0) {
        *current = elem;
        current->next = nullptr;
        current->index = countOfElements++;

        listOfStudents = current;
    }
    else {
        *current = elem;
        current->index = countOfElements++;
        current->next = listOfStudents;

        listOfStudents = current;
    }
}

void Group::findStudent(string str) {
    RecordBook *current = listOfStudents;

    do {
        if (!current->getName().compare(str) || !current->getLastName().compare(str)) {
            current->showScores();

            return;
        }
        else {
            current = current->next;
        }
    } while (current);

    cout << "Ничего не найдено" << endl;
}

void Group::insert(RecordBook &elem, size_t pos) {
    if (pos < 0 || pos > countOfElements) cout << "Неверный индекс" << endl;
    else {
        RecordBook *current = NULL;

        if (pos == 0) {
            current = listOfStudents;

            while (current->next) {
                current = current->next;
            }

            current->next = &elem;
        }
        else if (pos == countOfElements) {
            current = new RecordBook;

            current = &elem;
            current->next = listOfStudents;
            listOfStudents = current;
        }
        else {
            RecordBook *next = NULL;
            current = listOfStudents;

            while (current->next && current->index != pos) {
                current = current->next;
            }

            RecordBook *item = new RecordBook;
            next = current->next;

            item = &elem;
            item->next = next;

            current->next = item;
        }

        countOfElements++;
        updateOfIndex();
    }
}

void Group::erase(size_t pos) {
    if (pos < 0 || pos > countOfElements - 1) cout << "Неверный индекс" << endl;
    else {
        RecordBook *current;

        if (pos == 0) {
            current = listOfStudents;

            while (current->next->next) {
                current = current->next;
            }

            delete current->next;
            current->next = NULL;
        }
        else if (pos == countOfElements - 1) {
            current = listOfStudents;
            listOfStudents = listOfStudents->next;

            delete current;
            current = NULL;
        }
        else {
            RecordBook *next, *prev = listOfStudents;
            current = listOfStudents;

            while (current->next && current->index != pos) {
                prev = current;
                current = current->next;
            }

            next = current->next;
            prev->next = next;

            //!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
            ***//Вот здесь срабатывает ошибка***
            delete current;
            current = NULL;
        }


    }
}

void Group::updateOfIndex() {
    RecordBook *current = listOfStudents;
    size_t acc = countOfElements - 1;

    while (current->next) {
        current->index = acc--;
        current = current->next;
    }
}