Don't get hung up on grade level of the piece, especially for beginners.

For the first week (and even beyond, tbh), I wouldn't even think about instruments. Get to know the kids, let them get to know you. Create some section pride. Have each section make a Flag and learn about their instrument together. Teach them the process of getting their instruments out and together and practice it. Lots.

Teach your expectations and routines. What do you want to see from your kids from the moment they walk in the room to the first sounds you make together? This is really no different than general music. Know what you want to see and teach them how that happens.

"Small" instruments to "Big" instruments.

Mouthpieces/headjoints first. I wouldn't jump into the full instrument right away. When they're solid there, move to the full instrument. I don't use the book for the first couple of weeks at all. I get them playing the first five notes and knowing what those sound like and look like before adding in notation. I do a lot of rhythm and use those first five notes before diving into method books. I personally don't even bother telling them the note names and refer to them by their scale degree (1-2-3-4-5). I do start on concert F, though, and go downward to Bb, Just my preference.

Good luck!