I may or may not be able to help. Offshore next Monday, and I'll be taking crude samples as a matter of course. Posting shouldn't be a problem as I'll have all the necessary containers, COSHH forms, etc., but you seriously need to consider if crude is the best thing to be handing round in secondary school. All sorts of nastiness is contained within, not least of all being its carcinogenic properties. There's also a very real danger of flash fires/"explosions" caused by static discharges, and I'd imagine that there is plenty of static build-up when 30-odd kids are all rubbing jumpers trying to see the black stuff.
As mentioned above, it would be much, much easier and safer to use black treacle, soy sauce, etc. to get the effect. Spray some WD40 in it to get the smell.