Anne, I've been training in AIKIDO for the last 8 yrs. and as I have observed the most effective way in teaching and especially giving out corrections is by showing the whole group or the class what are the usual mistakes and there respictive corrections rather than stopping each student you see that needs help . Let the student explore the technique first , show him/her the basics then let them try . And it's not really nice for a Sensei or a Sempai to say to the student that what he/she is doing is wrong but rather say it needs improvement or suggest that it will be much effective if he/she does it this way. Always keep in mind that every individual is unique in it's own way , so this certain technique could be easy for you to perform but difficult for others or the other way around.
One secret also in dealing w/ this kind of Sempai's is to always think positive.
Don't let him get to you.