Glad to see people light up when something new comes along but before going overboard we should remember this is a 4th grade project.
So continuing along a line mentioned before I would go with netural boyancy and two motors with seperate speed controls to stear left and right and mount them to a horizontal boom (operated by a servo) that can be rotated to point them up and down to provide up or down thrust. (Sounds like a simple Arduino project with comms).
This way your Heliums lift cancels out the hardware weight and the motors provide the Up, Down, Forward, Reverse , Left and Right to change direction.
You can either make the boom rotate all the way around to point the motors nearly 360 deg or reverse the engines which would allow you to make one forward and one reverse to turn the air ship almost on the spot.
Hope this helped, good luck.