Which is grammatically correct?

[Unregistered]

underclassman failure cards
or
underclassmen failure cards

 

[bhaisahab]

underclassman failure cards
or
underclassmen failure cards

I don't know what an "underclassman failure card" is, but, if it's a document of that name, the plural would be "underclassman failure cards".

 

[tedtmc]

underclassman failure cards (whatever that means)
You use the singular for adjective.

not a teacher