JMSS student here.
Do the best you can on the day and whatever happens, happens. If you want to pursue a STEM career, you can obviously still pursue it without going into a science school, so don't feel disheartened if you don't get in. You're not "dumb/stupid/retarded/etc.". The entrance exam is just an indication of your abilities in different topics at the time you take it. You could do crap now, but improve at lot by the end of high school. You could do well now, but struggle to keep up with VCE later on. Point is, doesn't matter where you go for high school, the end result of going to uni and having a career that you like is what matters.
So yeah, not sure how effective that "pep talk" was. But remember that I used to be in your shoes and I'm just speaking from hindsight. Good luck, have fun and take care of yourselves.