It's possible to learn to kitesurf without a trainer kite. but not as easy.
But i recon you haven't yet explored all the glory that is your trainer kite.
If you can fly without crashing. then take it into the water and try a little bit of close to shore body dragging. yourl find out pretty fast if you can infact fly without crashing or not.
jump onto a skateboard and give the same thing a go. that will be amusing. its easy to fly a kite and easy to operate a skateboard.... but doing both at the same time is alot harder. same with kite surfing.....
try some gusty wind 15-25 knots,
try flying it in 1 knot.
take it out in 30 knots..... yourl find out how much power a little 2.8m can
produce!. infact if your really good yourl be able to fly a 2.8 in 40 knots. (your not that good don't do it!)
do some skudding. (skidding down the beach powered by the kite.) you should be able to endlessly skid down the beach changing direction at will if your skilled enough. If not skilled enough yourl stop and start constantly.
try some pendulum jumps. yourl need to a precise technique to jump on a trainer kite. again possible in enough wind but difficult.
lie down on the ground and use the power of the kite to pull you up.... then us it to slowly lower you down again. try and be completly straight... sorta like kite planking.
PS wear some safety gear, helmet, knee, elbow pads if your going to take on those challenges. you will be punished trying them.
$100 or so beans is a small price to see if you will enjoy and take up the sport. Heaps of crew throw away $1000's of beans because they buy all the gear, take big kite lessons and then discontinue for whatever reason.
Sure, you can book trainer kite lessons but this doesnt capture the reality of finding random time to persue a new sport. Totally agree with some other comments, the skills you learn on a trainer transfer directly to big kites and speed up the learning process significantly. And, yep, if you're single it makes an awesome excuse to catch up with that chick you just met and get to see her in a bikini and make her laugh 
i got my first trainer at 14, it lifts and separats to make me feel like a
real ....ohh opps wrong forum 
+1 for the skateboard, stay upwind, change direction without taking a foot/feet off etc...
I know i shouldn't do this but, comon guys if you are going to troll a thread with stuff like beans and trainer at 14, at least troll like a pro.
I still think your better off jumping on a smaller kite like a 7 or 8 in 15 knots if your 80 kgs plus. That way you can see how the kite behaves. Trainer kites arent a real representation of the kind of power your dealing with. I think everyone needs a little fear put into them. a few face plants....heck its supposed to be an extreme sport..just make sure its not an extreme sport for the spectators too....So choose your location with great consideration. ;)
And you end up with high wind kite once you've learnt.
I learnt on an 11m Fuel in 15 knots. Thankful for it too. Made me a better kiter IMO.
Now go fly your trainer in the water, preferably where you can't stand and there are currents and or waves. Can you keep it in the air whilst getting dumped by a wave? THAT is what your first few sessions with a board and a real kite will be like.
