Since I primarily have white and blue belt students, I have a yellow belt that I'm hoping to grade for green this summer teaching kihon every Monday. I give her the lesson plan of what I want her to teach, however, I primarily don't supervise her teach anymore. I won't be there tonight to watch her, and I do this for a reason...so she can learn under pressure and get the experience to eventually become a good shodan.
My sensei in the past gave me this responsibility (I taught almost all the children's classes when I was a teen) and I honestly believe it was a great learning tool for me. Years later, in Muay Thai, I became the assistant instructor and went through an "apprenticeship," that has made me a capable trainer. I'm not the brightest bulb in the shed and I truly believe that if it wasn't for being forced to teach, I wouldn't have half the knowledge I have today.
I don't think anybody here can argue that teaching is the best tool for learning.
So, with that being said, my students will give each other helpful advice when they're training, doing kumite, etc. They never give me advice but I wouldn't get upset with them if they did
I'd either agree or tell them why I don't agree... I encourage my students to ask me questions! I will usually stop the class when I'm teaching something and ask them as a group, "why do we do this?" "why do we do this like this and not like this?" because I want them to think and UNDERSTAND the hows, whys, wheres, and whats sooner, then later!
If you don't allow your students to freely ask questions, offer advice (even if they're wrong), it will take them much longer to learn...much longer.