index.ts

 onChange(): void {
    this.form.get('studentNumber').valueChanges.subscribe(number => {
   //assumed that student number must be 5 digits
      if (number.length === 5) {
            this._service.studentInfo(number).subscribe(response => {
              this.data = response;
            });
          }
      }
      else {
        this.data = {} as StudentModel;
      }
    });
  }