For the most part, kids have a hard time focusing at the end of the year (as do I - hello summer vacation!) but they are also really trained in the routines of the classroom, and know what is expected of them. So far, my experience with end-of-the-year subbing has been good, with just a couple students having issues.
Today I taught 1st, 2nd, 3rd and 4th grades as a roving sub. Essentially, the building hires 3 or 4 subs to cover each grade level for an hour or so, to provide the teaching teams time to plan for next year. It's not too bad of a gig - the plus side is if you get a hyperactive class (like the first graders I had), you are only there for an hour. However, if you have a nice class, like the 2nd, 3rd and 4th graders I had, you wish you could stay longer with them.