Yes. Unlike actual employment, there are no protections for students. Ie the school does not have to allow you to pick up where you left off, or refund your fees. I imagine you would get incompletes and have to repeat all of those classes.

That being said the military will often entertain requests to defer mobilizations to allow you to finish a semester. Not always, but I would say more often than not. There are a lot of students in the reserves, they don't want to **** them all off.