What will happen to me if my unit is scheduled to deploy before I go to school to reclassify?

One of two things can/will happen.

You will either stay behind with Rear D and then TDY En Route to your Reclass course (most likely to happen) or you will go on the Korean rotation but come back prior to everyone else so that you can TDY En Route.

My upmost recommendation is to talk with your Command Teams.

It's up to your unit if they want to keep you back or send you forward. If you're kept back, you should be on the Rear D UIC. If you go forward your unit will need to return you in time to complete your levy packet, get your orders, and clear.

That's roughly 8 months from the deployment start date to when you detach. In the Marine Corps you would deploy for a portion of it unless there was a reason to move you to a remain behind element. Work with your Unit and sort out the logistics. You might deploy you might not but work with them. The unit deploying doesn't cancel your orders to school. Relax.