Random numbers plus assigned number

Hi all,

I have a group of 172 students. I want to be able to assign a student a teacher color and the vlookup will put that student into that teachers sheet. Which I did. However, if a student doesn't get an assigned color I want to randomly generate a number and have that student go into a teachers sheet.
Sound confusing!
For example:
Any student that gets their cell highlighted green (colorcell=4), then they go into Mr. Wilson's class. Plus Mr. Wilson also takes any kid whose random number is between 1 and 20.
I have attached the sheet so you can get a better idea.

Thanks