At A-level you're expected to
work out what the positives and negatives might be, rather than simply to find them conveniently listed somewhere.
Take a look at pages like this one
http://mmjordahl.com/2013/04/26/descriptive-vs-prescriptive-grammar/
and this one
http://www.polysyllabic.com/?q=navigating/intro/prescriptive
and ask yourself how, in a debate, a supporter of descriptivism might champion his/her cause and how a supporter of prescriptivism might offer counter-arguments.
As a stating point for you though, I'll suggest that a 'pro' for prescriptivism is that it offers rigid structures to students of English, which seek to ensure that the language remains consistent and intelligible even over extended period of time. A 'con' against prescriptivism is that it 'straitjackets' the language by seeking to impose inflexible rules which fail to cover all possible circumstances or to recognise that English is a developing, rather than stagnant, language.