Apex Microtechnology has some good high voltage, high slew rate op amps.
If you only need to drive a square wave though, you can probably get away without using a specialized high voltage op amp. You can use a half-bridge gate driver with a floating high side driver, like TI's LM5109A or SM74104. You would use it to drive two N channel MOSFETs in series that would switch between a fixed positive voltage rail and ground, essentially giving you a square wave output. It should have no problem going from 0V to 50V in 100ns. This is essentially a buck converter (with synchronous rectification) without the output LC filter.
If you need to switch between a positive rail and a negative rail, you should use an isolated half-bridge gate driver, which is similar to a regular half-bridge gate driver, but the outputs are isolated and tied to sources of the MOSFETs they are driving. You would need isolated power supplies.
One thing to be aware of when using a half-bridge design is shoot-through current. This occurs when both MOSFETs are conducting at the same time, essentially shorting V+ to ground, which can damaged the transistors. As long as there is adequate dead time between the high side turning off and the low side turning on (and vice versa) you will be OK.
You might be able to find a evaluation board for a half-bridge gate driver that will do exactly what you need if you dig around.