Principal James Berardi assumed the envelope was from a parent. After all, who else would have left it for him, wedged between his door over the weekend? On Monday morning, when the principal found it in his office at Grizzly Hill Elementary School in Nevada City, Calif., he absentmindedly opened it and read the note inside, he told The Huffington Post in an interview. Then it hit him — a parent hadn’t written the note. It wasn’t a complaint or suggestion from a colleague. It was an apology letter from a former student, along with $300 that the student had…