I have plenty of school memories as a kid but no first days. The only first day I can remember is college. Getting out of a bus or something and looking for the dorms with my suitcase. Weirdly I don't remember my first day at another uni a few years later.
Oh, I remember it quite well. I lost my bag with all my books. Instead, to attend classes, I kept searching the whole school for it. Then I came back to see that my classmates had just learned how to write the first letters of the alphabet. I was so angry. Haha
I remember I stabbed a kid with a protractor in his hand, the first day at a new school in 3rd grade, he thought it would be funny to reach into my lunchbox without permission. So crazy to think about the fact how it was brushed off back then as boys being boys.