驗證選擇字段角2

Question

我正在使用angular2@2.4.1版本。我如何在field上做validation？

<div class="form-group" [class.has-error] = "schoolError"> 
      <label class="control-label" for="lang">Select School</label> 
      <select class="form-control" name="school" 
      required #school 
      [(ngModel)] = "model.school"> 
       <option value="default">Select a school</option> 
       <option *ngFor= "let sch of school">{{sch}}</option> 
      </select> 
     </div> 

<button class="btn btn-primary" 
    [disabled] = "form.invalid" type="submit">Submit</button> 

基本上我想禁用按鈕時，select field是無效的？如何選擇字段invalid未選擇任何值時？