HarmonyOS鸿蒙Next中为什么子类中用super.schoolname会打印undefined，用this.schoolname就没事

HarmonyOS鸿蒙Next中为什么子类中用super.schoolname会打印undefined，用this.schoolname就没事

这是我的代码：

class Inschool {
  protected id:string
  name:string
  gender:string
  protected schoolname:string
  constructor(id:string,name:string,gender:string){
    this.id=id
    this.name=name
    this.gender=gender
    this.schoolname = '大学'
  }
  protected sayHi(){
    console.log(`大家好，我叫${this.name}`)
  }
}
class Student extends Inschool{
  t_or_s:String = '学生'
  constructor(id:string,name:string,gender:string) {
    super(id,name,gender)
  }
  sayHi(): void {
    console.log(`大家好，我叫${this.name},学号${this.id}，是${this.schoolname}的一名${this.t_or_s}`)
  }
}
let xiaoming:Student = new Student('001','小明','男')
xiaoming.sayHi()

console.log(大家好，我叫${this.name},学号${this.id}，是${this.schoolname}的一名${this.t_or_s})这句代码在打印台显示

但是将this.schoolname改成super.schoolname之后，打印台输出undefined

请问这是为什么呢