Cadets is a weekly Sunday event where master raiders Silver & Cara train people with the commonly used tactics for all raids currently available in game. These are the hard mode tactics that are usually used for guild raids, and will stand you in good stead for most pugging opportunities, and give you a solid basis for a raiding career.
Make sure to read Silver's full post about the Cadets as it explains everything you need to know:
again? wasnt it an account issue last time aswell?
Swtor Twitter: We are seeing an account issue after today's maintenance and are going to delay bringing the servers back online to address it. Hoping it should be a quick fix but no ETA right now. Stay tuned for updates.