Help me Help Students! (Solar Powered, Switched, Red and Green Light Device)
I'm sorry, but you're on the wrong end of how things work. Let's start with sunlight.

Very bright sunlight (directly overhead) has a visible power density of about 500 watts/sq meter, and a brightness (illuminance) of about 100,000 lux. A common standard for indoor lighting is about 250 lux. As a rough number, this suggests that normal indoor lighting has a power density P of $$P = \frac {250}{100,000} \times {500} =1.25 \frac{watt}{m^2} $$ This isn't quite right, but it's close enough for a quick analysis.

With a box size (and therefor a solar cell size) of 2" x 4", this produces a power at the solar cells of $$P = .05 m \times {.1 m} \times {1.25} = 6.25 mW$$ The current efficiency champion for silicon PV cells is a bit less than 20%, so the maximum electrical power your box can produce under fluorescent lighting is on the order of $$P_e = 6.25 mW \times 0.2 = 1.25 mW$$ Assuming you can get a photocell whose maximum power point is at an output voltage of about 3.5 volts (to match your green LED), this suggests that the best current you'll get is $$i = \frac{P_e}{V_f} = \frac{1.25 mW}{3.4 volts} = ~0.37 mA$$ You'll note that this is about 0.5% of the maximum current, and while it will probably be visible I'm afraid it probably won't be very impressive.

As a sanity check, here is a solar cell about 40% bigger than your nominal box, which puts out a nominal 6 volts at 50 mA. Applying the brightness factor derived before suggests a classroom output of .125 mA. Agreement to a factor of 3 is pretty good for this kind of analysis.

Since a solar cell's output current is the important factor, Houston Fortney's circuit (with solar cell instead of power supply) will work as well as anything, and you can even get rid of the resistors.