Robin Thicke wasn't really dressed for it, but ... a bunch of high schoolers let him stand in for a photo anyway ... ON PROM NIGHT!
It went down last Saturday ... the 20 or so students from Eagle Rock High in northeast L.A. were waiting for a party bus to take them to prom when one recognized Alan Thicke's hunky pop-singer spawn shooting a music video across the street.
They called the "Blurred Lines" (No. 1 on iTunes this week!) singer over ... and he was more than happy to chill and take photos.
"He made our night!," said a student named Jesse. "When my other friends told me about their party buses, they really had nothing to say… I mean, we had Robin Thicke!”
The teens say Thicke asked if anyone wanted to appear in his video the following day ... we're told some of them took him up on it.