Don't do a class demonstration.
There are many public liability question that you need to consider and
running a Tesla coil at school is fraught with many problems.
Consider this scenario. You leave the class room for a few moments on a
bathroom break. Someone else starts playing with it and a junior school
kid touches the potential lethal parts with both hands.
Does the teacher know that it is that dangerous and is prepared to
notify the school's liability insurers to make sure it is covered.
There is a significant risk to school computers, PA and lighting.
I could go on.
A Tesla coil is great fun but a classroom is too dangerous (including
for me with many years of experience).
Do it at home with parental assistance/supervision and have a great time.
If you are running it indoors at home then a small wire Faraday cage is
your RF earth and grounded to mains earth.
A 3 foot diameter roll of fencing wire would be suitable. eg
http://youtu.be/b2ENAwH66PU