Re: It's getting annoying..
If you had mat time outside class when you could play with people on somewhat your level this situation might not feel as a burden, the way it does now. It could be that your teacher doesn't see the consequences of this lack of off class mat time. Teachers often follow the system they were formed in, without much reflektion. They might not see how practical differences in their own dojo as opposed to the one where they were trained should be adapted to also makes changes in dojo culture/class structure etc necessary. One class a week geared at higher levels maybe could be an idea? that will give you the energy you need for helping with the beginners.
In your dojo, do you usually train with the same partner the whole class? Introducing partner rotation, and dividing the duty of taking care of the newbes between the more advanced students in the group, could be a solution. There are flip sides to the no partner change-system too, but this is one of the major downsides. It's much easier to be the nice and caring sempai if you don't have to fulfill that role all the time.
I do suggest talking to your sensei.
I much prefer dojos where people are supposed to help out and think of the dojo as a whole, and not at every occasion grab the partner that suits them best. However, everybody also needs to be a little selfish - but too much selfishness ruins the dojo. There's a balance.