The first week is loud, and you want a book that does some of the work for you — one that gets a room of strangers laughing, or tells a child with a knot in their stomach that the knot is normal, or says out loud that everyone in here belongs. These are those books. Seven of them are picture books you can finish before the bell, and any single one does the whole job. Read them at whatever level the room is at. The eighth is a novel, and it's a different sort of commitment entirely; I've said so plainly below rather than slipping it in.
These eight books do three different jobs, and it will save you time to see which is which — a class does not need all eight. The first job is a laugh on day one, which is We Don't Eat Our Classmates: a T. rex eats her classmates, repeatedly, and it's very funny. The second job is first-day nerves, and three books here do it. The Kissing Hand is for the youngest and the most homesick, because it's really about leaving a parent. First Day Jitters is the twist ending that makes a room gasp. School's First Day of School is the distinct one — it's told from the school building's point of view, and the building is the nervous one, which lets a child who's scared to walk in be the brave one for a change. Pick whichever fits, then move on. The third job is telling a new group they belong, and again three books do it. All Are Welcome and Our Class Is a Family are warm, rhyming, spoken straight at the children, and either works. The Day You Begin is doing something sharper and lonelier: it sits with the child nobody in the room looks like, and it holds that loneliness for real pages before it lifts. If you have that child, it isn't interchangeable with the other two. Pick one from each group and you have a strong first week from three books. Then there's Wonder, which is not like the others at all — see its own note.