How to find sulfites in dried fruits?
The colorimetric method using rosaniline and formaldehyde is more sensitive and it is well documented. 

A somewhat safer version of the test (avoiding use of highly dangerous $\ce{HgCl2}$) is documented at MDPI - Open Access Publishing:


  "A concentrated pararosaniline stock solution (3.088 × 10 −3 mol/L) was made by dissolving pararosaniline hydrochloride in a 10% aqueous ethanol solution. The concentrated stock solution was used to make... [a] working solution contain[ing] pararosaniline hydrochloride, hydrochloric acid (HCl) and formaldehyde in the molar ratio of 1:1255:125 respectively. For example, a working solution with PRA concentration of 4.68 × 10 −4 mol/L in 25 mL contained 3,790 μL of concentrated PRA stock (3.088 × 10 −3 mol/L), 1,439 μL of 32% HCl, 189 μL of 36% formaldehyde solution and 25 mL with water."


Pararosaniline is available from Sigma-Aldrich, though it seems a bit pricey at ~$US82/25 g; perhaps your school would obtain some for use by yourself and by other students, as you would need perhaps a gram for your experiments.