I think you are talking about writing to a GPIO and that being received by the Arduino if so the Pi is 3.3v and the Arduino is 5V so you will need a "Level Shifter" to boost the voltage to 5V so the Arduino can read the signals.
Other than that there is no problem with doing what you are doing, apart from its a lazy solution and you won't score very highly. I know you don't want to mess with protocols but if you can use I2C from the Pi then use that to send messages to the Arduino. Your protocol could be as simple as 1 byte saying which pin you want to light up, you'll get better marks.
Edit
To use I2C is really quite simple because someone has done almost all the work for you in Wire.h, so just include that file and then to send a byteits simply a case of:
Wire.beginTransmission(device_address);
Wire.write(data_byte);
Wire.endTransmission();
Reading data sheets for devices makes it look a lot harder with all the transmission sequence diagrams, but you don't need to worry about that its all done for you. The hardest bit to remember is the pull up resistors on the data lines.
Have a look at these links for more information: