Do you need more than LEDs? AdaFruit, and others, have strips of addressable RGB LEDs
. Watch out for drive timing requirements. Some may not be directly controllable from RasPi because of tight timing control.
These are strings of shift registers with PWM drive output. To address one LED you write data to them all with the data for the one modified. The electronics for each node is simple and cheap.
You could use 74HCT595 8 bit shift registers. Search the forum for posts on those, and similar.
The standard method on stage is, I believe, to use DMX controllable stage lights. I think DMX is like MIDI, a serial protocol, but aimed at lighting rather than sound. 200 lights is going to be expensive.
I guess that for a school production you have a rather limited budget to buy 200 bright controllable lights.
You could also consider USB relay boards. USB provides an extendible addressable bus. The relay board typically appears as a serial port.
There are lots of 8 channel relay boards out there (e.g. this
) which should be viable and will cost about <£5 per channel, including USB hubs & cables, excluding lights and power.
I believe USB 2.0 can support 127 devices, so if we count hubs, and 8 channel relay boards you can have one 7 port hub feeding seven 7 port hubs from each port = 49 ports for relay boards = 392 lights with 57 USB devices.
I don't know if Raspbian=Pi will support 49 serial ports, but you could use more than one Pi.
I would go for the LED strips if possible (bright enough). You could fit split ping-pong balls over the LEDs to make your 'golfball-sized' lights.