Many Idols do the bare minimum or even drop out of school since they focus on their Idol career.
If they are rich they may get private tutors.
Since they use to be very busy and start their career early. not many do further education like go to University. If they do, they usually choose related lessons like media, dance or singing.
Some go to University when they are in hiatus or after they end their idol career.
So i guess in general idols are less educated than the average population.