Hell, most adults have no idea regarding intricate meanings of scripture.
Could this child have misinterpreted an answer he was given? Sure. Or perhaps he overheard adults speaking of the crucifixion, and in his childish naivety drew a picture thinking he was doing something good and/or acceptable. It certainly doesn't seem to be something running rampant among the youths so I'm going to assume this is a chance occurrence, nothing to get excited over.
I think that if he that man did spend most of his life under a rock and did not know about the be whole Jesus being nailed to a cross thing then he could have asked a another teacher before acting like a douche and sending the child home and requiring the child to go see a quack over a measly picture.
"A nation can survive its fools, and even the ambitious. But it cannot survive treason from within. An enemy at the gates is less formidable, for he is known and carries his banner openly. But the traitor moves amongst those within the gate freely, his sly whispers rustling through all the alleys, heard in the very halls of government itself. For the traitor appears not a traitor; he speaks in accents familiar to his victims, and he wears their face and their arguments, he appeals to the baseness that lies deep in the hearts of all men. He rots the soul of a nation, he works secretly and unknown in the night to undermine the pillars of the city, he infects the body politic so that it can no longer resist. A murder is less to fear"
Cicero Marcus Tullius
I'm sure the teacher knows what a cross is. I'm sure that has no bearing on a special needs child with a history of drawing violent pictures having presented his teacher with a torture scene labeled with his own name when asked what he did for thanksgiving.