I agree the English translation is pretty confusing. Assuming we're not looking for a vague sentence to begin with, and take this for what it's worth but we'd sooner hear any of the following: "The boy CAN see under your skirt", "The boy sees UP your skirt", "The boy sees YOUR KNEES under your skirt", "The boy sees (some thing or object) under your skirt". Now don't go thinking I'm perverted in any way :)
This is a strange sentence. To be able to SEE something under her skirt the boy will first has to LOOK under her skirt.
But what if there is some sort of reflective surface on the ground? Then the boy might inadvertently "see" under the skirt while having his head in a normal position.