The ongoing situation in Ukraine is a complicated topic for middle school and high school students in current events courses. The following short videos do a nice job of providing an overview of the key points in understanding the current situation. For a text-based overview of the situation, take a look at this timeline from the BBC.
Keith Hughes offers a good overview for high school students.
John Green, in his typical fast-paced style with a dose of snark, offers an overview in the video below.
The BBC provides a 60 second overview of Crimea and its significance in the current situation.