Arnold Schwarzenegger -- Son Graduates High School ... Arnold MIA

Arnold Schwarzenegger's son, Joseph Baena, donned a cap and gown Thursday as he graduated Frontier High School in Bakersfield, California ... but dear ol' dad was MIA.

Mildred -- Baena's mother/Arnie's former maid -- posed for pics with her son, who proudly displayed his diploma.

As for Arnie's whereabouts -- he was busy having lunch with his daughter Katherine in Santa Monica on Thursday.

Still, there's an argument to be made for him not showing up at Joseph's graduation. It almost certainly would have taken the spotlight off his son. Plus, who knows what Joseph actually wanted.

Either way ... congrats, kid!