Note that the graphics module isn't compatible with OpenGL ES 2.
You can use ES 2 with the window module, but can't use texture, shader, sprites, etc.
The prefix is there to ensure no symbol conflicts when someone uses their own GLAD headers
You also can't just replace the headers SFML is using. OpenGL ES 2 has done away with the fixed graphics pipeline which SFML is still relying on. As such you'll have to change quite a few things internally to "make it work".
There are a few ports that have more or less hacked something together, for example:
https://github.com/Zombieschannel/SFML/tree/GLES2