Heavy Metal Toxicity
This is probably a more complex problem than your instructor intended. In addition to the various metals having different toxicities, different compounds of these metals have different toxicities. 

For example, nickel metal has low toxicity, but the compound $\ce{Ni(CO)4}$ is a poisonous, flammable, volatile liquid. 

Likewise alkyl mercury compounds, e.g. $\ce{(CH3)2Hg}$ are vastly more toxic than the low solubility mercury salts like $\ce{Hg2Cl2}$.

If you just want to know about the pure metals, then this can be easier. Look up a Safety Data Sheet for each metal. It would be better if you pull all your SDS from the same source, that way they are all using the same data sources.

For example, here is a link to the SDS for mercury from Fisher Scientific. Section 12 contains ecological information, including LC50 (concentration that kills half of the population in the time period given) data for various aquatic organisms. Section 11 contains toxicological information, including LD50 (dose that kills 50% of the population) for various organisms. 

The best way to do this is to try to find one set of data for all the metals with the same organism for the best comparison. Then, devise your scale.