During class

When the teacher is giving an explanation then everyone sits or stand on one side. If you stand, make sure you stand up right and do not lean on someone else or on the wall. Sitting is done in za-zen or with your legs crossed. It is very rude to point your lower legs towards the sensei!
Make sure you move to the side quickly if the sensei is about to explain something. Training time is precious and time wasted can never be used again. Pay attention to what is shown; make sure you pay special attention to the footwork because every technique starts here.

After every explanation by the sensei you find a new partner. You try to train with as many partners as are present in the dojo. There may be people that you don't like or people with whom you don't like to train but that's not a valid excuse for not training with someone.
Nevertheless, you have to pick your partners with care. Find a high ranking belt if you think a technique is difficult or perhaps a less experienced person if you want to try something hard. Find partners that are difficult to work with so you can learn to work under difficult circumstances. This cannot be achieved by training with an easy partner.
Variety, that's one of the main points!

Normally you train in pairs. The one executing the technique is called Tori; the one receiving the technique is called Uke. Tori and Uke perform a role-play. Always be aware of this role-play and try to act accordingly. Uke takes care of a proper attack and stays alert during the entire technique.
Practise techniques both on the right-hand side as the left-hand side. After, for example, four turns you change role. If you work in a group of three (e.g. when there is an odd number of students on the mat), then one Tori defends against two different Ukes and you take turns in changing roles. (You do not work in a 'circle'!)
When training a technique the highest graded student always starts in the Tori-role. Always listen to a higher grade. The higher grades are among the people resonsible for the safety of the lower graded students training on the mat.

Prevent unnecessary conversation during class. Donīt teach each other. That's the sensei's job. Ask the sensei for advice if something doesn't work. Do try to work it out on your own at first.
Don't be afraid to make mistakes. The dojo is the right place to make mistakes, not the world outside it. Making mistakes is essential for effective learning.
Respect each other. Maintain that respect if people make mistakes, before class or during training. Learn from other people's mistakes. They can sometimes make mistakes that you couldn't even imagine!

Do not leave the tatami without the sensei's permission.

Always be careful with each other. There are a lot of techniques that can lead to serious injuries if used improperly!
Make sure you and your partner have a clear place on the mat. Make sure that breakfall and rolling techniques are executed towards the outer side of the tatami. This prevents ukes from falling on or rolling on one another.
Do not interfere with other people's injuries. The sensei will call for your help if needed. Keep on training if possible.