Just a note, I believe the word you are referring to is sempai...and personally, I tend to avoid it outside of a strictly Japanese context. Western society doesn't really allow for the development of the sempai / kohai relationship. It tends to become easy to abuse even in the Japanese context, let alone outside of it.
So use the words "senior" and "junior", which is all that "sempai" and "kohai" are used to mean in many (most?) dojos. I don't think the problem that Maggie is describing really has anything to do with either a sempai-kohai relationship or its absence. The problem is a simple one: some people take it upon themselves to instruct others as soon as they feel (correctly or incorrectly) that they have a grasp of something. Why
this is a problem for someone has many possibilities, not least of which the sting to the pride of being instructed by a junior. That and many other aspects are something that you can let slide like water like a duck's back if you so choose...or you can choose to let it bother you. For others, the solution is personal and local, so it's not really possible to offer useful advice except...seek out the personal and local solution.