I've experienced 3 types of dojo set-ups:
1. They mix beginners and advanced students together in one class
2. They mix beginners and advanced students together in one class but sensei teaches different techniques to both groups
3. The beginners, intermediate and advanced classes are totally separate.
Add to this:
4. basics classes oriented for beginners but welcoming all students, so that there are generally some more senior students on the mat for the newbies to partner with
5. mixed classes, period - everybody works on the same stuff, and again folks make sure newbies team up with more experienced students
I prefer these models because I believe it is helpful for newbies to partner with more experienced students and for anybody who has trained long enough to have somebody be his junior to to partner regularly with beginners, peers and seniors.