I'm reading various picture books to little children in elementary schools.
In that elementary school, there is a sixth grade boy who is a Koichi wannabe.
I’ve had feelings for you since I was in elementary school.
You've seen 学校 from a couple levels ago. Now we're adding a small to it. What is a "small school"? That's a school for small people, making it an elementary school.
The reading is the regular on'yomi reading of 小 plus the vocab word 学校 that you've seen previously. Combine these two things for the reading of 小学校.