hmmmm...... well it's a glitch, and maybe once the kid comes home try pausing the game once the kid comes out and it will look like he's holding somthing, then pause and click the invisible homework....see if that works, i never tried it!

Are you sure it's invisible? Often they come in the house and put it down somewhere you can't see very easily. What I do it as soon as the bus pulls up bringing them home, go onto that sim and cancel the "go to school" or "get off the bus" action, or whatever it says. Then they put the homework down out the front of the house.

Hope that helps!