A voltage divider only works well when you have a fixed load on the center.
The op amp will not always draw the same current. You need to be using a voltage regulator. Going from 24 to 12V you may need a heat sink on the regulator.
The input of an ordinary 7812 voltage regulator must be at least +13.5V for it to regulate the output at +12V.
The 7912 negative regulator needs a minimum input of -13.5V. Then your raw power supply must be at least 27V, not only 24V.
Then you need a circuit to divide the input voltage which also needs additional voltage.