。
Our class is made up of forty boys.
Literal
We [of-の] class [topic-は] male-students 40 [counter-人] [from-から] is-composed.
からなる ('to consist of, to be composed of') takes から ('from') to mark the components. A formal structural description — the class 'comes from' 40 boys. ~ている marks the state of composition. 男子 is a collective term for 'males, boys' (commonly students).