弄了半天，总算OK了。总是犯低级错误，总有中文符号想害我！

 var mydate=new Date();

  var weekday=["星期日","星期一","星期二","星期三","星期四","星期五","星期六"];

  var month=["1月","2月","3月","4月","5月","6月","7月","8月","9月","10月","11月","12月"];

  var scoreStr = "小明:87;小花:81;小红:97;小天:76;小张:74;小小:94;小西:90;小伍:76;小迪:64;小曼:76";

  var str=scoreStr.split(";");

 //成绩是一长窜的字符串不好处理，找规律后分割放到数组里更好操作哦

  for(var i=0,sum=0;i<str.length;i++){

    var myarr=new Array();

    myarr[i]=str[i].slice(3);

    sum=sum+parseInt(myarr[i]); 

  }

  var myavg=sum/str.length;

  document.write(mydate.getFullYear() +"年"+month[mydate.getMonth()]+mydate.getDay()+"日"+"&nbsp");

  document.write(weekday[mydate.getDay()]+"--班级总分为:"+ Math.floor(myavg));