I hope your teacher knows more about oncology than PowerPoint!
You would have to use code to fix the slides
On a COPY of the file
ALT f11 should open the VB Editor
INSERT > Module and paste in this code
Code:
Sub fixOnc()
Dim osld As Slide
Dim oshp As Shape
For Each osld In ActivePresentation.Slides
For Each oshp In osld.Shapes
If oshp.Fill.Visible Then oshp.Fill.Visible = False
Next oshp
Next osld
End Sub
Run by pressing f5
Does that fix it?