Woah. What should they know? How about everything! That's why it takes seven years to cover it all. O.W.L.s had a lot of stuff from first-year classes. He should know. He just took them.

He sat and thought about it for awhile. What should they know...? Well, I guess I can just say my favorite parts. He raised both hands into the air as usual. "Professor, I think first year students should know that Charms can be super fun! Like there's charms that will make your teacup tapdance or do cartwheels. If I had known that, I would've looked forward to Charms the most as a first year." Then again, for those practical people, maybe they just got scared off. "But Charms can also be dead useful. Like there's a charm to produce a jet of water, and if your robes catch on fire during Potions, no one will deny how handy it would be to know that incantation."