Potassium metal: Are we allowed to demonstrate potassium metal in schools? How do I deal with it if it has oxidised?

Potassium metal: Are we allowed to demonstrate potassium metal in schools? How do I deal with it if it has oxidised?
Hi,
I'd like to get ideas/techniques on how students can measure the amount of rust produced when a nail (iron) corrodes.
We are doing the rusting of steel wool with vinegar experiment with our Year 8 students.