T.I. -- School's a Bitch ... So I Got One for My Kids

T.I. arrived at LAX moments ago with an adorable wittle bitty puppy dog in his hand ... seriously, this thing is frickin' cute.

And when we asked why such a tough guy was cruisin' around with such a teeny tiny pet -- he explained that he promised his kids a puppy if they got good grades in school.

The kids came through ... so dad kept his word.

We also asked T.I. about the mega-party he attended at Justin Bieber's house the other day -- but the rapper made it clear, his lips are sealed.