This is most likely not really a chemistry question but actually about schooling. When someone is taking chemistry classes they attend both lectures and lab periods, and getting a seat on one does not necessarily mean you get a seat in the other.
It is possible then for someone to first have a semester just of chemistry lectures and then for the next semester take the associated labs. Their lectures and labs are offset from each other or “staggered”. That is what I would interpret someone to mean when they say they are “staggering in chemistry”, not some kind of chemical reaction or experiment.
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