I may just be an elementary school student, but if you underestimate me, you'll regret it!
When I opened the classroom door, there was a grade schooler sleeping like a rock.
When I was an elementary school student, I always made my decisions by flipping a coin.
You know 学生 is student, so what is a small student? Think young. That would be an Elementary School Student.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.