Here is a compiled list of the suggestions I got for 1st Day of School
titles:

First Day Jitters by Julie Danneberg (top recommendation)
Butterflies in My Stomach and Other School Hazards by Serge Bloch
How I Spent My Summer Vacation by Mark Teague
Thank you, Mr. Falker by Patricia Polacco

Side note--First Day Jitters is available as an ebook through Follett which
might be neat to use so the students could see the pictures better. It's
only a dollar more than print.