Do anything, however bad or dirty, and still get away with it without having to face the consequences or be punished
His looks are so deceptively charming, that he could get away with murder!
Explanation: His looks are so deceptively charming, that he could do anything he wants and still may not be suspected or punished for his deeds.
Getting away with murder, figuratively, means committing a murder and not being caught. However, in usage, it is about doing anything outrageous or forbidden.
In the example, the subject is "Deceptively Charming"; that is, he looks charming but in reality, he may be quite the oopposite of what he looks. Still, because of his looks, he could "get away with murder".
The teacher is so partial towards and supportive of those two students that the two of them could get away with murder.