Take a look at the original sentence again:
Several students sat sneezing and sniffling in their
seats, Jason always wore too much cologne,
and allergic reactions kicked into high gear as the odor wafted through
the classroom.
You wanted to fix it this way:
Several students sat sneezing and sniffling in their
seats, and Jason always wore too much cologne,
and allergic reactions kicked into high gear as the odor wafted through
the classroom.
Adding a coordinating conjunction like and will fix a comma splice. When you use this option, however, you must use it in a logical fashion. And shows addition. This connection device, though, makes no sense in the example above. The students were sneezing and sniffling because Jason was wearing cologne. You need to show cause and effect, not addition.
You might want to consult the rules for fixing comma splices and fused sentences.
Go back to the sentence to try again.