Is the school district in Jamsil not good???

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There is so much talk about the Jamsil school district being mediocre.
Considering the housing prices, it's hard to believe the school district could be bad.
Is it considered bad only because it's being compared to Daechi or Mok-dong?
Or is it actually poor overall?
For example, I'm curious how it compares to Junggye or Bundang, which are among the top 5 in the metropolitan area.