Rubidium (Rb) is the second most reactive (stable) alkali metal. It is so reactive that it instantly reacts with oxygen in the air to form a non reactive Rubidium Oxide (Rb2O) layer on the outside of the material. read more
The reason that this is banned in schools is due to the fact that Rb represents a significant increase in reactivity from the previous alkali metal, Potassium (K) due to the distance of it’s outermost electron shell from the neucleus (2,8,8,8,1) as oppposed to Potassium’s (2,8,8,1). read more