Calculate the average score of groups and each student, C
Completely prototype functions: debt() student_score() group_score();

// void debt();
void debt(int student_list_size);

Avoid naked magic numbers.  Use #define or a constant to self document various numbers.

#define TEST_N (15)
struct STUDENTS {
  ...
  char test[TEST_N];
};

void debt(int student_list_size) {
  // wchar_t test_wchar[15];
  wchar_t test_wchar[TEST_N];
  ...
  // mbstowcs(test_wchar, student_list[i].test, 15);
  mbstowcs(test_wchar, student_list[i].test, TEST_N);

Use size_t for array index types and the result of sizeof.  int may not be the right size to use as an index.  (Also i in main())

 // void student_score(int student_list_size) {
 void student_score(size_t student_list_size) {
   ...        
   // for (int i = 0; i &lt; student_list_size; i++) {
   for (size_t i = 0; i &lt; student_list_size; i++) {

Localize variables.  This approaches a style issue, but declaring variables close to their use has value.

void student_score(int student_list_size) {
  // int exam;
  // int average;
  // int digit;
  // int counter_digits;

  for (int i = 0; i &lt; student_list_size; i++) {
    int exam;
    int average;
    int digit;
    int counter_digits;

The strdup() serves no purpose.  Eliminate it.

// sscanf(strdup(buffer), "%d ...
sscanf(buffer, "%d ...

The return value of sscanf() should be checked and string widths limited.  Note: student_list[i].name will have a ' ' at the end.

// sscanf(buffer, "%d %[^0-9] %d %s", &amp;student_list[i].group, ...
if (4 != sscanf(buffer, "%d %99[^0-9] %d %14s", &amp;student_list[i].group, ...) {
   Handle_Format_Error();

Use valid main() signature.

int main() {
int main(void) {

Conversions char to/from wchar_t may need different sizes.
The extra '\n' in puts("4 - Выход\n"); looks wrong.  Guess I am not a fan of puts().  Why not use 1 function call?

fputs("\nВыберите номер пункта меню:\n"
      "1 - Cтуденты, имеющие задолженности\n"
      "2 – Средний балл каждого студента\n"
      "3 – Средний балл всей группы\n"
      "4 - Выход\n"
      "\n", stdout);