Robin Williams Went to Bat for 'Doubtfire' Costar After She Was Expelled

Robin Williams really was the nicest guy in Hollywood ... 'cause one of the child stars from "Mrs. Doubtfire" says he stuck up for her ... this after she got expelled as a teen.

Lisa Jakub -- who played one of Robin's onscreen daughters in the 1993 comedy -- stopped by fellow 'Doubtfire' alum Matthew Lawrence's "Brotherly Love Podcast" this week -- where she shared how she was kicked out of school while filming their flick.

She explained she was attending high school in Canada at the time -- but had to be on set to shoot the movie, which meant she'd be out of school for a good while ... an arrangement she says they were cool with at first, until they weren't ... and told her not to come back.

Lisa says Robin caught wind of this when he sensed something was bothering the kid -- and when he learned what was going on ... he took it upon himself to write a personal letter.

The note got sent to the principal of Lisa's school -- and she says Robin explained why he thought they oughta give her a second chance and reconsider expulsion, noting she was pursuing her career and that some flexibility was warranted.

However, not even Rob's star power could save her ... as Lisa says they didn't budge and that she remained expelled -- not ever going back to that particular high school.

With that said ... she claims the principal ended up framing the letter as a cool memento.