Try burning turpentine.
It produces a very soot rich flame, per my recollection.
Another path per the article: Preparation of Carbon Nanoparticles from Candle Soot.To quote:
Carbon nanoparticles (CNPs) were prepared by simply mixing candle soot into ethanolic Nafion solution under ultrasonic radiation for 90 min. Well-defined CNPs could be obtained after centrifugation and washing.
[EDIT] My colleague mentions other choices for a school lab, citing benzene as a non-choice. The reason, let's hear from what the CDC says about it, to quote:
Immediate signs and symptoms of exposure to benzene.
People who breathe in high levels of benzene may develop the following signs and symptoms within minutes to several hours:
Rapid or irregular heartbeat
Death (at very high levels)
Another option suggested is toluene, comments from Toluene Safety Tips from MSDS experts – MSDSonline, to quote:
Toluene is a highly flammable liquid and it can cause mild damage to the skin and the eyes. However, the most-common hazard associated with this chemical is inhalation. Products containing toluene can produce dangerous fumes which can cause nausea, headaches, unconsciousness, and even death if inhaled.
Note, my final choice, for high school labs in general, is safe ordinary candle soot with a reference citing a preparation path to nanoparticles. Now, I could make other more potentially efficient and likely problematic suggestions, but I sleep well with this conservative/responsible choice.