I shit you not, go with Pokemon.
My school was pretty "fuck you" towards school assemblies in general, so I've got a fair idea on what your cohort is like. If you go up there and just be like "yo, please try hard" - it's going to be something they've heard before.
Point 3 for Amalgam I think is key. Take a look at the motivational rants I write (would be awesome ULP practice, just btw) -- there's always a connection of some sort. "At this time of year you feel like x" - to which the response will be "YES! YES! I DO FEEL LIKE X!"
But it's different with your cohort. I'd begin blase (however, I'm a speaker unlike many), sort of lulling them into a false sense of security, and getting them to laugh more than anything else, so they hardly expect your speech. Enter: Pokemon.
"You know, VCE is sort of like Pokemon. Everyone sort of has this underlying wish to be the best, but Team Rocket just said "dicks on you" and dug a trap hole called 'the ceebs'."
And so for a good portion of your speech they're having a giggle, and you've been working in some empathetic sort of "and it feels impossible" sort of stuff, and then you twist at the end and smash out the cliche stuff "Yo, it's possible, I did it <insert motivational message>"