You won't get sent back to Raleigh. Your training in Collingwood is your trade training, teaching you the theory about the equipment you are likely to working on once you get onboard. If you fail any modules you will likely get backclassed and need to retake the module and exam. If you fail more than a couple of times you may be required to change branches to a less technical trade, that's if they Navy retains you at all.
There are only 2 critical modules for WE trade training:
1. Learning how to use the phone to callout contractors when your kit breaks and scream "HEEELLLLLPP!"
2. Learning how to sleep anywhere, anytime in any conditions for 20 hours a day, waking only to go to scran, particularly watchkeepers midnight scran during defence watches.
Fail either of these and it's civvy street for you.