"First thing: Do not get disarmed. If you get disarmed, it doesn't matter how many spells you know, you can't cast any of them. If you do get disarmed, your best bet is to duck."
"Now, the Disarming Charm is Expelliarmus, ex-PELL-ee-ARE-muss. We'll go around the room to make sure you can all say it right. [First kid]?"
"Here's how you remember it: expelli and armus. Expelli liked Expelled; to be taken out, be it out of school or out of your hand. Armus like Armed; if you lose your wand, you're not armed anymore. Ex-PELL-ee-ARE-muss, everyone got it?"
"Good. Now pair up, you and you, you and you, and you and you. If you really hate your partner, come up here and I'll give you a new one. If not, go form a line down the middle of the room. Stand a few feet apart, facing your partner. Start working on the spell. We'll start off easy, so for now, don't hold your wand too tightly."
"First thing: What did your previous professor teach you last year? And how much of it do you remember after the summer?"
"Defence Against the Dark Arts isn't about how many counter-spells you know. There's an infinite number of ways to attack you, and you can't learn an infinite number of counter-spells. So you need to think fast, and be creative."
"To learn that, you're going to spend a lot of time dueling in this class. On the board, there will be a list of attacking spells you may use when dueling. If there's a spell you would like added to the list, speak to me about it before or after class. If anyone uses a spell that's not listed, detention is a minimum; depending on the spell in question, you may even get expelled. Do I make myself quiet clear?"
"Don't unpack! Just sit there for a few minutes until everyone shows up. You can chitchat."
"Alright, everyone here? Since it's double DAtDA today, we're going to the Forbidden Forest to do some Dark Creatures work."
Whatever they come across.
"You're teenagers now, so your common sense is fading, so this lesson is going to be about common sense."
"Do not provoke people! Do not start fights! If other people are fighting, don't get in the crossfire!"
"Don't go out on full-moon nights! Be polite to merpeople, centaurs, and goblins! Don't go to Greece, there are a ridiculous number of dangerous creatures there."