I doubt anyone needs or wants a synopsis of the Harry Potter universe, I'm sure we're all familiar with one of the most popular series ever written. Furthermore, I doubt you'd choose this story if you weren't, so I'm gonna keep this first addition simple. This story will go from year to year in order, feel free to use knowledge from the books or movies.
Finally after making your selection for what year to follow you can come up with a few scenarios, guaranteeing that everyone has the opportunity to see what they want from any given pairing. No rules against multiple shrunken witches and wizards, shrink the whole school except for one if you want. As long as one character remains bigger than the rest.