running a clinic

[Andy]

Are they "fair to weak" even though some have been umpiring for a long time?

I'm going to take some liberties here and read between the lines, so to speak.

If you have an established group of umpires and the majority are as you say, I take that to mean that you have a group that is mainly just there to pick up some part time work and is not really interested in getting better. Or, it could be that this group has just not had any real quality training in the past.

It sounds to me as if your best bet is to split them based on years of experience, since that is a concrete number that nobody can argue with.
If you use "level of umpiring" or "quality", those are very subjective and you could ruffle some feathers by how you place people in certain groups.

At this level, I would recommend sticking to very basic umpiring topics. Mechanics, signals, how to be and look like an umpire.

If I am way off base on my assessment, I apologize...just what I am picking up from your post.