有两处错误：
1、sql语句，inner join 表，怎么会是c.GradeId = 变量名？
2、SqlParamater para 变量体，声明变量@ClassName是多余的，完全没有用到。
实际上只需要将@GradeId变量定义一下就可以：

/* Frm窗体 */
int gradeId = cboGrades.SelectedValue;
string className = txtClassName.Text.Trim();

string sqlStr = @"
select ClassId, ClassName, GradeName, Remark"

sqlStr +=" from ClassInfo a
inner join GradeInfo b on a.GradeId = b.GradeInfo"

sqlStr +=" where 1=1"

SqlParameter[] paras =
{
    new SqlParameter("@GradeId", SqlDbType.Int),
    new SqlParameter("@ClassName",SqlDbType.NVarChar,20)
};
if(gradeId >0)
{
    sqlStr +=" and a.GradeId = @GradeId";
    paras[0].Value = gradeId;
}

if(!string.IsNullOrEmpty(className))
{
    sqlStr += " and ClassName like '%' + @ClassName + '%'";
    paras[1].Value = className;
}

//调用DAL层方法
DataTable dtClasses = SqlHelper.GetDataTable(sqlStr, CommandType.Text, paras);



/* SqlHelper DAL层方法类 */
public class SqlHelper
{
        //定义连接对象
        private static SqlConnection _conn = null;

    /// <summary>
    /// SqlDataAdapter执行查询方法，支持存储过程
    /// </summary>
    /// <param name="cmdText">StoredProcedure,512/TableDirect,4/Text,1(默认)</param>
    /// <param name="cmdType">sql对象</param>
    /// <param name="para">参数组</param>
    /// <returns>DataTable数据集</returns>
    public static DataTable GetDataTable(string cmdText, CommandType cmdType, params SqlParameter[] para)
    {
            _conn = new SqlConnection(ConnContext);
            DataTable dt = new DataTable();
            SqlDataAdapter sda = new SqlDataAdapter(cmdText, _conn);
            if (para != null)
                    sda.SelectCommand.Parameters.AddRange(para);
            sda.SelectCommand.CommandType = cmdType;
            sda.Fill(dt);
            sda.SelectCommand.Parameters.Clear();

            return dt;
    }
}