With iPhoto going away and Photos replacing it in Yosemite, many teachers have become a bit lost. One recent question on Staff Talk was how to take the images from Photos 1.0 and use them in iMovie 10.7. Although exporting isn't required, this method does make it easy to find all of your images in one place to easily add them to your movie.
Keep in mind that Photos now enables you to create a slideshow movie right in the application itself (without using iMovie). So, if you are just creating a slideshow, stay tuned for my follow up post about that aspect of photos.
I had a teacher ask me if there was a way to hide or remove students who are no longer in your class. The answer is maybe. It depends on if they are officially dropped from your class and appearing as red in your roster and gradebook. If not, then no, you cannot remove them until they have been unenrolled from the class. If the student is showing up as red in your gradebook though, you can hide them so you don't have to worry about them. Here is a screenshot to help you do it: