LEDs with a built in resistor is good for quick and simple lights but it might be better to teach a student the reason for a current limit resistor on an LED in the first place, or they're going to be confused the first time they plug a common LED into 5 volts and it goes up in smoke. Ohms law does not strictly apply to a diode as it's a semi conductor device and VERY non linear, the exact forward voltage and how fast the current increases after that point is going to depend on manufacturing variations and the ambient temperature, mind you this means that when the diode starts conducting it's going to heat up and it's forwards voltage drop is going to change.