How do I get a specific tag in XPATH?

Two questions:

• The element is p
  
  bound to the XHTML namespace. You are referencing an element named "p" in "no namespace". Declare an XHTML namespace and use the namespace prefix in XPath.
• The predicate (square brackets) behaves like an SQL where clause and is a filter for the item preceding the square brackets. Your original XPath tried to address all nodes text()
  
  from any element in $data
  
  that has a child p
  
  , instead of selecting all nodes text()
  
  from all xHTML p elements.

(: declare a namespace prefix for the XHTML namespace, 
   in order to use in the XPath below :)
declare namespace x="http://www.w3.org/1999/xhtml";

(: $data example :)
let $data :=
  <div xmlns="http://www.w3.org/1999/xhtml">
    <div class="title">my title</div>
    <div class="course">
      <div class="room">112</div>
      <div class="teacher">Mr. Wilson</div>
      <div class="student">123456</div>
      <p>approved</p>
    </div>
  </div>

return
  for $i in $data
  (: use the namespace prefix "x" when addressing the HTML p element 
     and pull it out of the predicate
   :)
  let $ptag := $i//x:p/text()
  return $ptag