The 10 Rules of Duke Jiu-Jitsu
1. The first rule is to cut your nails.  All 20 of them.
2. YOU are responsible for not injuring your training partner, even if they are being stupid and not tapping.
3. No twisting leg locks (i.e., heel hooks) or kneebars.  The ONLY submission on the legs you are allowed to go for is a straight achilles lock, and if you don't know what that is, ignore the legs entirely.  If you want to try other leglocks, you need to ask permission from one of the instructors AND make sure it is OK w/ your partner when rolling.
4. No small joint manipulations (e.g., no trying to break a finger), no eye gouging, no strikes, no biting, no scratching, etc. Most other things that won't cause injury are legit -- e.g., I can drive a forearm or hand into your throat. Only neck crank someone if you are in their guard.
5. No slams.  If you can pick someone up and set them down gently from greater than waist high, then you win the match no matter what the person on the bottom was doing.  If this happens to you (i.e., you get picked
up), learn the defense.
6. There's no pressure to do takedowns / strikes / mma. Also, get a mouthguard.
7. If you don't know how to fall, do not start on your feet.  Consult w/ one of the wrestlers or judo people if you want to get better at this.
8. Take a shower as soon after class as you can.  Skin infections suck. Also wash all clothes you roll in -- they'll harbor bad things otherwise.
9. Try to train with everyone: that includes people bigger or more in shape, or better than you. Technique can make up for a lot, and small people often learn
faster as a result (b/c they are denied the brute force option).
10.  Wear a pair of board shorts that won't come off your body.