How to get current node's children using XPathSelectElements in ASP.NET C#?

I am facing a trouble for a couple of days. I have an XML string which is retrieved from a web service. I want to list them into my webform.

My (very very simplified) XML string looks like:

<school>My School</school>
    <department>Logistics</department>
    <department>Trading</department>
    <department>Food Control and Quality Analysis</department>
<school>Her School</school>
    <department>Poultry</department>
    <department>Forestry</department>

I use the following code to create a treeview:

XDocument xdoc = new XDocument();

xdoc = XDocument.Parse(service.getTree());

var SchoolNames = xdoc.XPathSelectElements("//school");

foreach (XElement sn in SchoolNames)
{
    divContent.InnerHtml += string.Format("<h4>{0}</h4>", sn.Value);

    var Departments = sn.XPathSelectElements("//department");

    foreach (XElement d in Departments)
    {
       divContent.InnerHtml += string.Format("<h5>{0}</h5>", d.Value);
    }
}

I get the following result:

<h4>My School</h4>
    <h5>Logistics</h5>
    <h5>Trading</h5>
    <h5>Food Control and Quality Analysis</h5>
    <h5>Poultry</h5>
    <h5>Forestry</h5>
<h4>Her School</h4>
    <h5>Logistics</h5>
    <h5>Trading</h5>
    <h5>Food Control and Quality Analysis</h5>
    <h5>Poultry</h5>
    <h5>Forestry</h5>

which I should get

<h4>My School</h4>
    <h5>Logistics</h5>
    <h5>Trading</h5>
    <h5>Food Control and Quality Analysis</h5>
<h4>Her School</h4>
    <h5>Poultry</h5>
    <h5>Forestry</h5>

In my opinion, the problem is on the line

var Departments = sn.XPathSelectElements("//department");

If a filtering criteria is written here then the problem will vanish out. If one can write here a "only departments of which ancestor is the current d" condition. But I do not know how to write it.