"So I'm late for the start – I'll just slip in here at the back.
.......I am going to have to get off the mat for...... ?"
Well from a 'western' health and safety point of view coaches need to control what is going on their mat and particularly during the class for someone to disappear off the mat leaves questions – are they injured, is there a problem? Coaches are responsible and must therefore divert their attention to establish what's going on. All the Lancashire Aikikai Coaches are first aid qualified and well able to assess health/injury issues and in fact have a duty of care to all.
So to find out someone has bobbed off the mat to go a get a drink, or go to the toilet is reassuring that nothing is amiss – BUT is disrespectful. It has also diverted the attention of the coach from the class to find what was wrong or send out a search party. Trust me people have been looked for!
The courtesy of telling your coach of your 'need' or personal reasons to leave the mat to my mind is polite and the thing to do. From a eastern point of view the dojo is a special place where rules and discipline exist alongside etiquette. It's part of the package of aikido. You can't just choose the bits you want to and leave other bits to one side. At the beginning of the class it is customary to say Onegai-shimasu, "Please help me in my practice", as one bows to the sensei; it's a bargain being struck.
The commitment at the beginning of the class starts by making sure you arrive in time rather than adopting the western 'just in time' that usually fails and you are late. if you are rushing are you then mentally focused? If you do happen to arrive late, wait by the edge of the mat until the coach grants permission to join the class.
To join the class late you need to blend and join in with what the rest of the class is doing after all you should have been there in good time. However sometimes the coach may ask you to warm yourself up away from the others you should follow any direction.
To join a class is to practise - but as one gets older (or recovering from injury) one may be less able to do the full range of techniques. One should try to modify to be similar but if this is not possible - does the teacher know your limitations? He needs to know that you are just not opting out, picking and choosing, but that there is a reason that you are sitting out. Our coaches can show adaptations of techniques which can be tried to keep you practising. But they need to know from you as might your partner. One simplistic example of adaption are kneeling techniques – if knees are bad standing technique can be adopted rather than not practising.
In small club classes coaches become aware of your limitations however on courses you can't presume a different sensei knows.
You should never be standing around idly or chatting on the mat... either be practising or if for reasons mentioned earlier you can't adapt then sitting quietly and unobtrusively at the side of the mat - until you are able to rejoin the class. This means you don't become a hazard to others or are distracting practise.
Andrew Baird Published 2015
Footnote: the protocol for courses can vary from the above.