Why don't Phineas and Ferb's parents ever notice what they're doing học home school?

They're not always clueless, sometimes their Dad knows what's going on.

As for episodes where they get caught; that might depend on your definition of "caught."

Here's a list of episodes where it's at least debatable:

Dude, We're Getting the Band Back Together
Mom's Birthday
The Best Lazy Day Ever
Phineas and Ferb Get Busted!
The Lake Nose Monster
Interview With a Platypus
Perry Lays an Egg
Thaddeus and Thor
Phineas and Ferb's Quantum Boogaloo
The Bully Code
Invasion of the Ferb Snatchers
The Lizard Whisperer
She's the Mayor
Wizard of Odd
Ladies and Gentlemen, Meet Max Modem!
The Secret of Success
Candace Gets Busted
Ask a Foolish Question
Bad Hair Day
Phineas and Ferb Interrupted
A Real Boy
Agent Doof
What'd I Miss?
My Sweet Ride
Backyard Hodge Podge
Love at First Byte