1. The Ministry of Magic's Muggle Liaison Office was short-staffed, and no one had read the Muggle newspapers all that week.

2. Who do we know who thinks all Muggle-borns are scum?

3. [Source] Alecto, Amycus's sister, teaches Muggle Studies, which is compulsory for everyone

4. Rowling's The Tales of Beedle the Bard, where in his commentaries on the tales, Albus Dumbledore reveals that a witch some centuries ago tried to Bowdlerise the eponymous tales to rid them of realistic dread and pro-muggle messages.