Q:

Name for this rhetorical device

The text: "The court determined that Student B exhibited adverse educational impact because notwithstanding her passing grades, during her final year at School #1, her symptoms were sufficiently severe that she was unable to attend public high school at all and required homebound instruction."
My analysis: The distortion is in the word "passing." Student B was actually earning all A's and B's. But the author of "the text" finds it expedient to call the grades "passing" because elsewhere in her document she argues that Student A did not experience a significant academic decline when he went from a 3.8 gpa to a 2.2. So she harps on the fact that although Student A's grades have been declining, he is still passing his courses... and therefore should be found ineligible for special education.

A helpful ELU participant proposed the following description of what's going on:

When a fact is presented as representative of the truth, even though it clearly omits key details.

What is this rhetorical device called?
Bonus question: I also need an effective idiom or simile which describes this form of bias or slant.

A:

This involves a willful violation of the Gricean maxim of quantity (eg 'truthfully' saying that you can see a cat in the garden when you can actually see 10, and you're not in a logic class). The misrepresented 'quantity' here is the quantity / measure of marks / grades being (mis)represented. 
The intent to mislead by (for instance) using ambiguous phraseology is termed
obscurantism 

the practice of deliberately preventing the facts or full details
  of something from becoming known.
  {ODO} (bolding mine)

.......

deliberate obscurity or evasion of clarity. {RHKWebster's}

