Re: Beginners with delusions of grandeur...
Senior students are allowed to make *basic* corrections at my dojo - "don't let your hands get behind your head" for shihonage, for example.
If the sensei is walking by, though, I'd rather wave them over than say something myself; for one thing, the sensei will often notice and correct something totally different that what I wanted to say, and for another my shoulder injury came from trying to help a kohai with kaiten-nage and not paying enough attention to my own ukemi.
Far better for both me and my kohai if I concentrate on taking good ukemi and let the teacher teach.
In a bigger dojo, I imagine it would be much more important for the sempai to help out with basic corrections; more than 10 students is a big class for us, though, so we get a lot of personal attention.