> void main
main returns int

> char user_choice;
> while (user_choice!='z')
This variable is uninitialised at the point you first use it.

> university univ;
This isn't initialised either.
Which is very important when you get to make_file(), since you do
- dereference an uninitialised pointer
- try and realloc an uninitialised pointer

> if (un->ptr==NULL) un->ptr=(student*)realloc(un->ptr,(count+1)*sizeof(student));
The test serves no purpose, since at best it only extends the array once.

In main(), you need this to start off with a NULL pointer
university univ = { 0 };

And this function needs to be something like

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int make_file(FILE *in,university *un) {
  char buff[BUFSIZ];
  int  count=0;
 
  while ( fgets( buff, sizeof buff, in ) != NULL ) {
    student s;
    if ( sscanf( buff, "%s %ld %f %ld",
                 s.name, &s.id, &s.avg, &s.hw_submit ) == 4 ) {
      /* success decode, extend array and copy the info */
      void *temp = realloc( un->ptr, (count+1)*sizeof(student) );
      if ( temp != NULL ) {
        un->ptr = temp;          /* update array */
        un->ptr[count] = s;      /* copy data */
        count++;                 /* one more stored */
      } else {
        /* no more room, return with what we have */
        return count;
      }
    } else {
      /* that line didn't make sense, report it */
      fprintf( stderr, "Bad line %s", buff );
    }
  }
  return count;
}

int make_file(FILE *in,university *un) { char buff[BUFSIZ]; int count=0; while ( fgets( buff, sizeof buff, in ) != NULL ) { student s; if ( sscanf( buff, "%s %ld %f %ld", s.name, &s.id, &s.avg, &s.hw_submit ) == 4 ) { /* success decode, extend array and copy the info */ void *temp = realloc( un->ptr, (count+1)*sizeof(student) ); if ( temp != NULL ) { un->ptr = temp; /* update array */ un->ptr[count] = s; /* copy data */ count++; /* one more stored */ } else { /* no more room, return with what we have */ return count; } } else { /* that line didn't make sense, report it */ fprintf( stderr, "Bad line %s", buff ); } } return count; }