I can't see how the code you posted would give you that error so I think you're cutting down the code and missing out something important. If your Cls table has more columns that id and name, try
public JsonResult jsonClassList()
{
var classes = from c in db.Cls
select new {id = c.Id, name = c.name};
return Json(new { Classes = classes }, JsonRequestBehavior.AllowGet);
}