The song doesn't say either the son or the daughter are GT students, so the word "join" is just pinhead dumbassery.
Oh if I had a had a daughter sir, I'd dress her in white and gold.
And put her on the campus, to cheer the brave and bold.
But if I had a son sir, I'd tell you what he'd do
He would yell "TO HELL WITH GEORGIA!"
like his daddy used to do.
At what point are either students? At what point is the daughter not included? You can interpret the song as a dad taking his 6 year old son or daughter to a football game. Hell, I always kind of thought that was the gist of it anyway.