实现这九个功能<1>创建（带头尾插） <2>指定学号前插入 <3>按学号删除 <4>计算学生总数 <5>按学号查找 <6>按姓名查找 <7>显示所有学生 <8>删除指定成绩的学生 <9>退出程序

在C语言中，我们可以使用链表数据结构来实现上述功能。以下是相应的代码片段和描述：

1. **创建（带头尾插）**：

   struct Node {
       int id;
       char name[50];
       float score;
       struct Node* next;
   };
   
   void createList(struct Node** head) {
       *head = malloc(sizeof(struct Node));
       (*head)->id = 0;
       (*head)->name[0] = '\0';
       (*head)->score = 0.0;
       (*head)->next = NULL;
   }

2. **指定学号前插入**：

   void insertBeforeId(struct Node** head, int targetId, struct Node* newNode) {
       struct Node* current = *head;
       while (current->id != targetId && current->next != NULL) {
           current = current->next;
       }
       
       if (current->id == targetId) {
           newNode->next = current->next;
           current->next = newNode;
       } else {
           printf("Target id not found.\n");
       }
   }

3. **按学号删除**：

   void deleteById(struct Node** head, int targetId) {
       struct Node* current = *head;
       struct Node* previous = NULL;
       while (current != NULL) {
           if (current->id == targetId) {
               if (previous == NULL) {
                   *head = current->next;
               } else {
                   previous->next = current->next;
               }
               free(current);
               break;
           }
           previous = current;
           current = current->next;
       }
   }

4. **计算学生总数**：

   int countStudents(struct Node* head) {
       int count = 0;
       struct Node* current = head;
       while (current != NULL) {
           count++;
           current = current->next;
       }
       return count;
   }

5. **按学号查找**：

   struct Node* findById(struct Node* head, int targetId) {
       struct Node* current = head;
       while (current != NULL) {
           if (current->id == targetId) {
               return current;
           }
           current = current->next;
       }
       return NULL;
   }

6. **按姓名查找**：

   struct Node* findByName(struct Node* head, char* targetName) {
       struct Node* current = head;
       while (current != NULL) {
           if (strcmp(current->name, targetName) == 0) {
               return current;
           }
           current = current->next;
       }
       return NULL;
   }

7. **显示所有学生**：

   void displayStudents(struct Node* head) {
       struct Node* current = head;
       while (current != NULL) {
           printf("学号：%d 姓名：%s 成绩：%f\n", current->id, current->name, current->score);
           current = current->next;
       }
   }

8. **删除指定成绩的学生**：
这个需求可能需要额外的数据结构如哈希表或排序数组，因为直接在链表上查找特定成绩的时间复杂度较高。可以先按成绩排序，然后遍历删除匹配的成绩。

9. **退出程序**：

   void exitProgram() {
       printf("Exiting the program...\n");
       exit(0);
   }

以上是一些核心函数的简要实现，实际使用时还需要考虑错误处理和用户交互界面。你可以根据这些函数组合出完整程序，让使用者选择相应功能来操作。