Definitely go with the PI 3B ... its faster, its complete (bluetooth, wifi), and for the most part nothing really has changed that will affect education; however, always be aware that tutorials (on-line and otherwise) are dated. We just can´t get away from that concept.
I´ll give you a for-instance. The activity light on the 2B is tied to the GPIO pins and the student can toggle the light momentarily under program control. This will not work on the 3B... the lights have been moved to the other side of the board (to accommodate the on-board wifi antennae) and [the LED] is no longer tied to the GPIO.
Most of the difference you will find in on-line (and other) examples and tutorials between the 2B and 3B will have to do with the difference between wheezy and jessie raspbian. But, they are close enough that you´ll be able to figure it out.
The 3B is snappier, has on-board connectivity, and is the same price... its a no brainer really.