People have commented on this one a lot already but I gotta also add, this is just plain not the case. Even if the white belt/junior is coming at it from a lower skill level, you can have people who are very focused on kumite (or kata) tournaments (or just want to excel outside of a tournament environment) and want to get pushed to the limit and beyond to up their skills and ability. As long as they are doing so for the right reasons and are up to it (and most senseis and good experienced seniors will be able to determine that fairly quickly), seniors may well spar and train with them at a far higher level. Also, seniors may want to, and hopefully do, up the anty with juniors to see what they can take and are capable of...if things look a bit too much, then a good senior will of course throttle it down again (that should go without saying!!). It is a great school that realises that everyone, at all levels, is of different ability in different areas and brings different levels of aptitude with them to class. Again, I have always loved the tournament environment (that's just me) and when I crossed over to karate I started at white/yellow but wanted to be pushed on the competition/fight side of things and got served by the sensei that focused on fighting/competition as much as I could handle. I used to go home at night beaten to hell at first (which I was fine with) but I adapted quickly and my karate game improved exponentially from mixing it with those that had been there and done that. I think it is the wrong approach to just keep people of the same level primarily training with their peers, the only way to really up your game (be it kata, kumite or whatever) is to play and train with the "big boys".