Change 2nd Select Values Based On 1st Select?

I am working on a class registration system that requires students to register for a main class (101, 102 or 103). The student is supposed to select one main course as well as provide a second choice in case the first is not available. I have two dropdown select fields to capture data

1) Choice -1: 101 / 102 / 103 ( Student needs to select one - Reading the classID from classes table)
2) Choice -2: 101 / 102 / 103 ( If student selects 101 in Choice-1 then the only classes available under Choice-2 should be 102 or 103).

How can I accomplish the above? Further to this there are two fields on the form where I would like to auto populate based on what they have selected in Choice-1 and Choice-2.

For Example:
If a parent selects choice1: 101 the child Choice1 field should autopopulate with 100.
If a parent selects Choice2: 201 the child Choice2 field should autopopulate with 200